In 1982, he was charged with sexually assaulting a child, a crime that could have led to 15 years in prison. The case was strong, and the victim, a 12-year-old girl, was prepared to testify. But officials didn’t prosecute him for the crime. Instead, he pleaded no contest to a separate and far less serious charge of grand theft — and he served his sentence not in the state prison but in the Pinellas County jail, where he could continue working as an informant.

In one six-year stretch in the 1980s, Mr. Skalnik testified or gave information in at least 37 cases in Pinellas County alone, many involving murder charges. Most of those cases ended in convictions or plea deals, and four of the defendants were sentenced to death.

“Together, we never lost a case,” Mr. Skalnik once wrote to a judge.

Being a jailhouse informant is a good gig, if you can get it. Just about everyone who’s been locked up is aware of the benefits that come from snitching — sentences reduced, charges dropped. For the same reason, inmates quickly learn to steer clear of known informants like Mr. Skalnik, who often wind up in cells next to people who are awaiting trial.

Mr. Skalnik always denied that he was promised any sort of leniency for his testimony. But over and over again, he provided critical testimony in a murder case, and then, sometimes days later, his own sentence would be reduced or his prosecution would magically disappear. Just five days after Mr. Dailey was sentenced to death, Mr. Skalnik was released from jail for his assistance, even though he was facing a 20-year sentence of his own, and even though his parole officer had warned the year before that he “is and always will be a danger to society.”

The problem isn’t simply that Mr. Skalnik is an inveterate criminal, or that he repeatedly got special treatment from the authorities. It’s that, as is the case with many serial jailhouse informants, his incentive to tell stories that prosecutors wanted to hear was far greater than his incentive to tell the truth. And that has led to great injustices, like the one Mr. Dailey is living out.

Of the 367 DNA-based exonerations on record, one in five involved testimony from a jailhouse informant. A 2004 study found that snitch testimony was the leading cause of wrongful conviction in death-penalty cases.

It’s hard to gather more precise data on the use of informants, because prosecutors don’t keep such records, which puts defendants, and jurors, at a disadvantage. How are you supposed to judge the credibility of an informant’s testimony if you aren’t told that he has been feeding similar stories to prosecutors for years? And how are you supposed to weigh the break he gets from prosecutors if they don’t give it to him until after the trial?