Even though this type of testimony should be viewed through the most critical lens, it is still relevant and admissible at trial.
What is a jail house informant? An incarcerated person who testifies against another incarcerated person based on conversations the two had.
As a prosecutor, I frequently received letters from inmates telling me they had "information" on a case I was prosecuting. The correspondence was always in a handwritten envelope and letter from the correctional facility. The author provided me just enough details to pique my interest, but not enough to vet the accuracy of any alleged confession.
There were a few steps required when receiving a letter like this:
1) Check whether the proposed informant had access to the defendant I was prosecuting to even have the ability to obtain a confession.
2) Check why the proposed informant was in custody. Did he have an attorney because he had pending charges?
3) Set up a meeting.
4) Investigate the information.
I was always a hopeful skeptic at these meetings. No prosecution should be built based solely on the testimony of a jailhouse informant, but additional evidence never hurt. The key to these meetings was to obtain details of the conversations between the informant and the defendant. Details were the key. Anyone could come in and say, "Johnny told me he killed that guy." What did Johnny the defendant say about how he did it, about where he hid the evidence? I was looking for the details that were not reported in the media or not elicited in court. I needed details only the culprit would know.
Ninety percent of the time these meetings proved to be a waste. The informant either did not have enough information or I could not help the informant enough in their requests. This was a quid pro quo after all. If I wanted the informant's testimony, I needed to give him something. Did he want to plead to a lower charge? Have his case dropped totally? Commute his sentence? If I wanted the testimony, I could usually negotiate a plea deal for the informant. But I never did, and never wanted to, drop charges or attempt to get a person out of their sentence.
Another reason the potential testimony failed? Discovery. If the defendant had already received his discovery and his or her attorney had provided him a copy, how could I prove the defendant's confession came from his mouth as opposed to facts the informant put together from the police paperwork.
When a prosecutor makes a deal like this, there is always a stain on the case and even on the prosecutor depending on how good the deal was. The defense attorney would say, "Look at the evidence. Look at the witnesses. The informant holds a gun to a little old lady and takes her purse. He gets a slap on the wrist as long as he will say what the prosecutor wants." I only used jail house informant testimony if it would be window dressing on an already solid foundation of a case.
There is also a legal analysis required. A person has a Sixth Amendment right to have an attorney present for all questioning by authorities. In New York State, this right attaches, and cannot be waived, when a person is formally accused of a crime. Essentially, the informant cannot have been acting as an agent of the police at the time he obtained the information from the defendant. It does not matter that he was doing it purely in his own self-interest with the intention of going to the authorities. The only rule is that he cannot have been doing it at the direction of law enforcement. A judge is required to conduct a Cardona hearing to determine if the informant was acting as a law enforcement agent or not. The minute the informant approaches law enforcement and volunteers to tell them about his conversations with the defendant, he is an agent. Therefore, any conversations obtained following the informant reaching out to the prosecutor or police will be inadmissible.
Once an informant gets a deal for testifying against a defendant, the odds are the informant will appear just about every time they get arrested. They know that they can get lower charges and sentences just by getting information for the police. We used to call these guys professional snitches. They know the game, the system, and the laws so well that they have the ability to say whatever is necessary to save their own neck. It is an even lower form of evidence than an informant used one time.
A good defense attorney can destroy informants on cross and a prosecutor should never rest his or her case on only this evidence. But just because this testimony has drawbacks, doesn't mean it should never be used. As a legendary prosecutor in my old office once said in his closing argument, "If you play around in the sewer long enough, you're going to become friends with some rats."
By the way, the case that gave rise to that quote was reversed for prosecutorial misconduct for improper comments on summation.