ESCHBACH, Circuit Judge.
An F.B.I. informant accompanied a murderer on the night that the murderer killed the plaintiff’s brother. The issue in this case is whether that informant breached a constitutional duty owed to the murder victim and consequently caused the victim’s death. We conclude, based on the undisputed facts, that he did not and thus we affirm the judgment of the district court. It follows from this conclusion that the judgment entered in favor of F.B.I. supervisory officials must also be affirmed.
I.
The facts surrounding the murder of Jeff Beard have been litigated in at least two trials and reported in two opinions of this court. See United States v. Robinson, 503 F.2d 208 (7th Cir.1974), cert. denied, 420 U.S. 949, 95 S.Ct. 1333, 43 L.Ed.2d 427 (1975); Beard v. Mitchell, 604 F.2d 485 (7th Cir.1979). The basic facts have thus become fixed through litigation and time and are not generally disputed in the instant case. We will not again recite the colorful history of William O’Neal, an F.B.I. informant connected with Beard’s death; instead we will set forth the salient facts of Jeff Beard’s murder over ten years ago.
William O’Neal was considered by the F.B.I. to be an excellent informant. As such, his contact person in the F.B.I., agent Roy Mitchell, encouraged O’Neal to gather information about a Chicago police officer named Stanley Robinson. In April of 1972, therefore, O’Neal accompanied Robinson during several criminal episodes, including a homicide. O’Neal related Robinson’s activities to agent Mitchell, who naturally was skeptical about the claim that a police officer was a violent criminal. In accordance with F.B.I. procedure, agent Mitchell reduced O’Neal’s statements to writings available for inspection by his superiors.
On the afternoon of May 17, 1972, Robinson called O’Neal and told him that he would not see O’Neal that evening because of other plans. O’Neal protested that he did not wish to be excluded from any plans. Robinson then explained that his plans consisted of a small job of no interest to O’Neal. Nevertheless, O’Neal did persuade Robinson to include him in the night’s activities.
Robinson, accompanied by O’Neal, set out that night to perform a murder contract. Robinson only had a vague description of the target and only knew his first name— Jeff. After driving around searching for Jeff for some time, Robinson acquired some better identification information concerning Jeff. Robinson then returned to the search for 2Vz hours until O’Neal stated that he was tired and wanted to go home. While driving O’Neal back to his car, Robinson fortuitously spotted Jeff Beard in a pool hall.
Robinson and O’Neal sat in the car outside of the pool hall for 45 minutes. During that time O’Neal went to a telephone and tried to call agent Mitchell at his home. Mitchell was not at home and O’Neal returned to the car. When Beard finally came out of the pool hall, Robinson approached, arrested, and handcuffed him. Robinson and Beard got in the back seat of the car and Robinson ordered O’Neal to drive to the “Indiana District,” which O’Neal interpreted as an instruction to drive south. O’Neal drove south until Robinson, who wanted to make a telephone call, ordered him to exit from the expressway. O’Neal did exit and Robinson left the car for 3 to 5 minutes to make a call.
When Robinson returned to the car, he began driving and told Beard that he was not under arrest but that they wanted him to sell narcotics. After entering Indiana, Robinson pulled off on the shoulder of the road and asked Beard to get out of the car so that they could talk. O’Neal remained in the car. When Beard got out of the car, Robinson shot him; the wound was not fatal however and Beard raced across the road. O’Neal told Robinson that he had really “messed up.” Undaunted, Robinson pursued Beard across the road, caught up with Beard, and murdered him.
O’Neal called agent Mitchell early the next morning and reported the murder of Jeff Beard. Owing chiefly to O’Neal’s testimony, Robinson was subsequently convicted on charges of violating Jeff Beard’s constitutional rights, see 18 U.S.C. §§ 241, 242. See United States v. Robinson, 503 F.2d 208 (7th Cir.1974), cert. denied, 420 U.S. 949, 95 S.Ct. 1333, 43 L.Ed.2d 427 (1975). Robinson was sentenced to life imprisonment.
In 1976 Eloise Beard brought this action directly under the Constitution, see Bivens v. Six Unknown Named Agents of Federal Bureau of Narcotics, 403 U.S. 388, 91 S.Ct. 1999, 29 L.Ed.2d 619 (1971), for damages relating to the murder of her brother. Named as defendants in this suit were O’Neal, agent Mitchell’s supervisors in the F.B.I.’s Chicago office, and the acting director of the F.B.I. The plaintiff brought a separate Bivens-type suit against agent Mitchell.
While the instant case was pending, the action against Mitchell went to trial before a jury. The jury returned a general verdict in Mitchell’s favor; judgment was accordingly entered, and we affirmed, see Beard v. Mitchell, 604 F.2d 485 (7th Cir.1979). The district court in the present case subsequently cited the Mitchell litigation and ruled that res judicata barred the plaintiff’s Bivens-type claims against the F.B.I. officials and O’Neal. The defendants’ motions for summary judgment were accordingly granted and these appeals pursuant to 28 U.S.C. § 1291 followed.
II.
Under the res judicata doctrine, a final judgment on the merits bars further claims by parties or their privies based on the same cause of action. Montana v. United States, 440 U.S. 147, 153, 99 S.Ct. 970, 973, 59 L.Ed.2d 210 (1979). Mutuality of parties remains an essential element of the doctrine. Nevada v. United States, — U.S. —, 103 S.Ct. 2906, 2925, 77 L.Ed.2d 509 (1983). Res judicata bars the present claims against O’Neal and the F.B.I. officials, therefore, only if these defendants are in privity with agent Mitchell.
We can discern no basis for holding that all F.B.I. agents and informants, sued individually for their own acts or inactions, are in privity for res judicata purposes. See Restatement (Second) of Judgments §§ 43-61 (1982). To be sure, Mitchell and all of the defendants in this case are associated with the F.B.I. For res judicata purposes, however, we fail to perceive why this relationship is relevant and why a suit against one F.B.I. agent should be considered a suit against all people associated with the agency.
The case that the defendants cite in connection with their privity argument, Sunshine Anthracite Coal Co. v. Adkins, 310 U.S. 381, 60 S.Ct. 907, 84 L.Ed. 1263 (1940), provides no support once the case is examined fully and in its historical context. In that case the Court wrote:
Where the issues in separate suits are the same, the fact that the parties are not precisely identical is not necessarily fatal.... There is privity between officers of the same government so that a judgment in a suit between a party and a representative of the United States is res judicata in relitigation of the same issue between that party and another officer of the government.
Id. at 402-03, 60 S.Ct. at 916-17. This holding, however, is a description of the modern doctrine termed “collateral estop-pel” — once an issue is actually litigated, that determination is conclusive in a subsequent suit involving one party to the prior action. See Montana v. United States, 440 U.S. 147, 153, 99 S.Ct. 970, 973, 59 L.Ed.2d 210 (1979). At the time Sunshine Anthracite Coal Co. was decided, the term “res judicata” was still used to encompass the then emerging doctrine of collateral estop-pel, which does not require mutuality of parties. See 1B J. Moore, Moore’s Federal Practice ¶ 0.441[2] (1983). Sunshine Anthracite Coal Co., therefore, stands for the currently unexceptional principle of collateral estoppel that when a person sues a government official, all issues actually litigated are binding in a subsequent suit against another government official.
If agent Mitchell and the current defendants were sued only in their official capacities, then they would clearly be privies. Official-capacity suits are, in reality, suits against government entities — in this case, the F.B.I. See Monell v. Department of Social Services of the City of New York, 436 U.S. 658, 690 n. 55, 98 S.Ct. 2018, 2035 n. 55, 56 L.Ed.2d 611 (1978). Separate suits against F.B.I. officials in their official capacities, therefore, are suits against the F.B.I. and res judicata principles would apply. See Lee v. Peoria, 685 F.2d 196, 199 n. 4 (7th Cir.1982).
In the absence of privity, the defendants are still entitled to contend that some issues, litigated in the Mitchell case adversely to the plaintiff, bars recovery in the present case. Such an argument has some appeal; because agent Mitchell was found not liable to the plaintiff, it might seem to follow that his F.B.I. supervisors must also not be liable.
The jury in Mitchell, however, returned a general verdict in the agent’s favor. The jury may have found in Mitchell’s favor on one or more of several grounds: for instance, (1) that O’Neal did not breach a constitutional duty, (2) that Mitchell was not personally responsible for O’Neal’s acts, or (3) that Mitchell was entitled to qualified immunity. On appeal, we expressed serious doubt about whether Mitchell was personally responsible for O’Neal’s conduct and whether O’Neal was a proximate cause of Jeff Beard’s death. See 604 F.2d at 499-500. Given the posture of the appeal, however, our doubt was not translated into holdings. Thus, from reading the general verdict and our opinion in Mitchell, it is unclear what issue or issues were decided against the plaintiff. Rather than extend and apply collateral estoppel in this unclear area, we prefer to affirm the district court’s judgments on another ground — one that applies to the F.B.I. officials and O’Neal.
III.
The Fifth Amendment guarantees, among other things, that a person will not be deprived of life without due process of law. Jeff Beard had a constitutional right, therefore, not to be murdered by someone acting under color of federal authority. See generally Brazier v. Cherry, 293 F.2d 401 (5th Cir.1961). The plaintiff’s suit against O’Neal and the F.B.I. officials is purportedly founded on O’Neal’s violation of that right.
At various points the plaintiff alleges that O’Neal “participated” in the events that led to Beard’s death. Use of a vague word such as “participate,” however, does little to advance the constitutional analysis in this case. A witness or a victim might be termed participants in a criminal episode, yet we would not call them criminals. We must thus analyze O’Neal’s specific acts and determine whether this conduct breached a constitutional duty owed to Beard and, if such a breach occurred, whether it may have caused Beard’s death.
The plaintiff does not contend that O’Neal is responsible for Robinson’s acts in the way that a conspirator might be responsible for a co-conspirator’s conduct. It is undisputed that O’Neal did not want Beard killed. On at least two occasions O’Neal took steps to save the lives of people whom Robinson intended to murder. Moreover, O’Neal became associated with Robinson and was with him on the night of Beard’s murder only to obtain information for agent Mitchell. Before the murder, in fact, O’Neal attempted to telephone Mitchell to inform him of Beard’s peril; tragically, Mitchell was not at home. And shortly after Beard was killed, O’Neal informed Mitchell of the event. In no sense, therefore, can it be said that O’Neal directed or approved of Robinson’s conduct.
This case is unlike a situation where a uniformed police officer, who is in a position to prevent violence, observes a murder without intervening in any way. Absent an explanation for the officer’s inaction, it might be legitimate to infer that he approved of the murder. Indeed, the officer’s presence and authority might facilitate the murder by providing the symbolic support of the government. In such a case, the officer might be personally liable for the acts of the person who operated the murder weapon. The same cannot be said in this case, where O’Neal had reasons (e.g., fear for his safety and loss of his guise) not to take further steps to prevent Beard’s murder.
Nevertheless, O’Neal is responsible for his own actions. O’Neal’s decision to accompany Robinson on the night of the murder must thus be examined. In essence, the plaintiff contends that O’Neal had a constitutional duty not to be in Robinson’s presence on that tragic night.
We hesitate to hold that a long-term government informant has a constitutional duty to refrain from being in a position to witness crimes involving the deprivation of life, liberty, or property. One of the functions of an informant is to witness crimes so that an arrest can be made and future crimes prevented. Robinson was arrested and convicted in large part because of O’Neal’s decision to accompany Robinson; had O’Neal remained at home and Robinson escaped criminal liability, the number of Robinson’s victims undoubtedly would have grown. We need not rule on the constitutionality of O’Neal’s decision to accompany Robinson, however, because the undisputed facts reveal that there is no causal link between O’Neal’s acts (as distinguished from his inaction) and Jeff Beard’s death.
A plaintiff may not recover damages for a constitutional tort without establishing causation in fact — i.e., that the defendant caused the claimed injury. See Loss- man v. Pekarske, 707 F.2d 288, 291 (7th Cir.1983); Arnold v. International Business Machines Corp., 637 F.2d 1350, 1355 (9th Cir.1981); see generally W. Prosser, Prosser on Torts 244 (1971). In one constitutional tort case, Mt. Healthy City School District Board of Education v. Doyle, 429 U.S. 274, 97 S.Ct. 568, 50 L.Ed.2d 471 (1977), the Supreme Court held that the plaintiff could not recover if the claimed injury would have occurred anyway. A strict application of this “but for” test of causation would deny recovery in the case of joint causation, where either of two forces would have produced the injury independent of the other. It might therefore be appropriate in some cases to apply a broader notion of causation, one that will permit recovery in the case of joint causes. Under either a strict or broad definition of causation, however, O’Neal’s actions cannot be described as a cause of Beard’s death.
The record shows that Robinson murdered several individuals, without O’Neal’s assistance, prior to Beard’s murder. Robinson further made it clear that he intended to kill Beard on the night of May 17 without O’Neal’s involvement. Robinson did not seek or want O’Neal’s assistance. For purposes of gathering information, O’Neal insisted on going with Robinson on the night of Beard’s murder, but in no way did O’Neal’s presence significantly increase Beard’s peril. In fact, O’Neal’s presence provided a witness, a factor that would make many murderers hesitant. Moreover, O’Neal attempted to contact agent Mitchell to inform him of Beard’s danger, thus providing Beard an opportunity to survive that he otherwise would not have had. O’Neal’s actions can only be said to have caused Robinson’s arrest and conviction, not Beard’s death.
Our analysis on causation to this point is little more than a restatement of our observation in Beard v. Mitchell that “we have great difficulty describing O’Neal’s conduct as a proximate cause of Beard’s death without definitely holding that he had a fixed duty to prevent the crime.” 604 F.2d at 499 (footnotes omitted). Given the posture of that appeal, it was unnecessary for us to decide whether O’Neal’s inaction breached a constitutional duty and caused Beard’s death. We must now answer that previously unresolved issue.
In a sense, O’Neal’s inaction caused Beard’s death; if O’Neal had intervened and prevented the crime, Beard would not have been murdered. However, this is merely a tautological statement that could be made about anyone alive at the time of the murder. We each failed to prevent the crime and thus Jeff Beard died. We are all not liable for damages, however, because clearly we did not all have a constitutional duty to intervene. For the sake of clarity, therefore, our focus of attention must be on whether O’Neal had a constitutional duty to act to prevent the murder, not whether O’Neal’s inaction caused Beard’s death. See generally L. Green, Rationale of Proximate Cause 11—43 (1927).
We hold that O’Neal did not have a constitutional duty to prevent Jeff Beard’s murder. In Bowers v. DeVito, 686 F.2d 616 (7th Cir.1982), a case also involving the deprivation of life, we had occasion to discuss the Constitution’s guarantee of due process:
[Tjhere is no constitutional right to be protected by the [government] against being murdered by criminals or madmen .... The Constitution is a charter of negative liberties; it tells the [government] to let people alone; it does not require the federal government or the state to provide services, even so elementary a service as maintaining law and order.
Id. at 618. O’Neal’s failure to take steps to protect Beard may be considered “monstrous,” but such failure did “not violate the due process clause of the ... Constitution.” Id.; see Fox v. Custis, 712 F.2d 84, 88 (4th Cir.1983); see generally Jackson v. City of Joliet, 715 F.2d 1200 at 1204 (7th Cir.1983).
At oral argument the plaintiff attempted to turn Bowers v. DeVito around to support her case. The plaintiff argued that O’Neal’s role was not merely passive because he put Beard in a position of danger just as if he “had thrown him into a snake pit.” Bowers, 686 F.2d at 618. We cannot discover, however, any special relationship between O’Neal and Beard that gave rise to a constitutional duty to provide protection. Jeff Beard was a “member[ ] of the general public, living in a free society, and having no special custodial or other relationship with” the F.B.I. or O’Neal. Fox v. Custis, 712 F.2d 84, 88 (4th Cir.1983). Neither O’Neal nor any agent of the F.B.I. singled Beard out to be killed. Chicago police officer Stanley Robinson targeted, abducted, and murdered Jeff Beard. The plaintiff’s constitutional tort case is against Robinson, not O’Neal and the F.B.I. officials.
Finally, we note that this case is not brought under the Federal Tort Claims Act. Whether the defendants violated some common law duty is thus not an issue on this appeal. Our decision is not based on the common law of Illinois; rather we hold, on the basis of the undisputed facts, that O’Neal committed no constitutional tort that led to the plaintiff’s brother’s death.
IV.
The F.B.I. defendants may be held liable only if O’Neal committed a constitutional tort for which they were personally responsible. We seriously doubt that the F.B.I. defendants, who never had any immediate contact with O’Neal, could be found personally responsible. See Lojuk v. Quandt, 706 F.2d 1456, 1468 (7th Cir.1983); Crowder v. Lash, 687 F.2d 996, 1006 (7th Cir.1982). In light of our decision that O’Neal is entitled to summary judgment on the Bivens-type claim, however, we may affirm the judgment in favor of the F.B.I. officials without reaching the question of personal responsibility.
The judgments of the district court are affirmed.
. The F.B.I. was also named as a defendant but the district court properly dismissed it on the ground of sovereign immunity.
. For the purpose of this opinion, we will assume, without deciding, that O’Neal acted under the color of federal authority.