What happens if a person has evidence that an accused party is innocent, but to give that evidence would be self-incriminatory?

Question

Suppose that Person A is accused of a crime. Person B (unbeknownst to anybody but themselves) actually committed the crime. Suppose further that Person B is called as a witness in the case of Person A.

Would there be a way for Person B to make it apparent to the court that any conviction of Person A for the crime would be invalid, without simultaneously putting themselves on the hook for the crime?

I am asking this primarily in the context of the right to avoid self-incrimination. Could Person B say something along the lines of "I know for a fact that Person A is not guilty of this crime, but I refuse to explain why I know that, on grounds that it would incriminate me". Or, is there no way for the court to accept that Person A's conviction would be unsafe, whilst simultaneously respecting Person B's fifth amendment rights?

Answer 1

Person B, while on the witness stand, could only respond to valid questions. It is not likely that any question would permit the response "I know A is not guilty, but I can't tell you how I know because it would incriminate me." If B did say something of this sort, and was believed, the authorities would no doubt have strong suspicion that B was at least involved, and would investigate B. Since B chose to say that much, such an investigation would not be barred by the "fruit of the poison tree" rule. B would be taking a serious risk by doing this.

And of course, B might not be believed, and A might be convicted anyway.

Answer 2

There was a similar case in the USA (but reported on a computer site in the context of facial recognition, not because any law aspects).

Mrs. A stole things in a store. She was almost caught by two store detectives, but managed to escape. Shortly later, Mrs. B matching the description was arrested, the two store detectives swore she was the thief, and she was convicted.

A local newspaper reported about the case, including a photo of Mrs. B. Mrs. A saw the report, realised what had happened, and went to the police confessing the theft. Mrs. B was released. Unfortunately it was not reported what happened to Mrs. A.

The newspaper then also published photos of Mrs. A and Mrs. B. They looked absolutely identical. So the two store detectives couldn't be blamed one bit for identifying Mrs. B.

I would hope that a judge would give Mrs. A the most lenient sentence possible, because she (1) confessed without being a suspect at all, and (2) through her confession kept an innocent person out of jail.