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Say someone is on trial for a murder they committed. During the trial, one of the witnesses, a friend of the defendant, admits that it was actually they who were the killer. It seems that this would be enough to create reasonable doubt and get an acquittal, or even possibly have the state drop the charges because they now believe that they should be charging this other person.

After the trial ends, the friend presents irrefutable proof that they weren't actually the murderer. Of course, this friend could be charged with perjury, obstruction, etc... but I can't be re-tried for the murder because of double jeopardy, and my friend can't be tried (or won't be found guilty) because of the evidence that he didn't do it.

Is there a flaw in this system? Something that prevents someone from actually doing this?

Related: Can someone get protection under Double Jeopardy for a crime by arranging to be put on trial with fake evidence that is then disproven?

Side-note; something just like this happened in an episode of Law and Order; though I had been wondering about this long before seeing that.

It is not as simple as the witness just making the assertion that they are the killer.

They will be subject to grueling cross examination to break their story.

• If the victim was killed at a specific time, perhaps the prosecution can prove the witness was somewhere else at that time, and therefore lying. (No Opportunity)




• If the victim was killed with a specific weapon, perhaps the prosecution can prove that only the accused had the weapon, and the witness had no access to it, and therefore lying. (No Means)




• If the victim was killed in a specific way, perhaps the witness doesn't know any of the details of how the crime happened, and therefore is not credible. (No Knowledge)




• If the accused's DNA is found at the scene, and their shirt is covered in blood, and the witness has no corroborating evidence against them, then the witness is likely lying. (No Evidence)




• If the witness claims he is the murderer, the prosecution can inquire as to why he killed the victim. The real murderer had a reason: Perhaps money, power, hatred, passion, etc, that the witness may not be able to provide. (No Motive)

Planting doubt in the mind of a jury is an effective defense. But lying about who did what, when, how, and why it is not as easy as you suggest.

I think this sort of thing happens far more often in fiction than in real life. Off hand I recall a Perry Mason novel and a Donald Westlake novel which used versions of this plot. In the Parry mason the witness invoked the fifth Admendment, and only after having been granted immunity, was asked "Did you see the deceased that night" "Yes I saw him when I shot and killed him." Of course, being a Perry Mason novel, the real criminal was convicted in the end.

If this were attempted in actuality, it might be held that the Harry Aleman precedent applied, and that the accused had never been in jeopardy, and could therefore be retried (in the Harry Aleman case the judge was bribed). I don't know of any actual case where this argument against Double Jeopardy has been made.

In any case the witness who claims to have committed the crime would surely be guilty of perjury, probably of obstruction of justice, and possibly would be held to be an accessory to the original crime. The witness and the original accused might be charged with conspiracy to commit the original crime, or with conspiracy to obstruct justice.

Besides that, all the possibilities described in the answer by user abelenky might be used to show the witness was lying during the trial, and thus to convict the original accused.

In short this would be a quite risky scheme for both the witness and the accused. To the best of my knowledge it has rarely been attempted in actuality.