In the late 1980s, Stuart GraBois, a federal prosecutor in New York, had become obsessed with the disappearance of a 6-year-old boy named Etan Patz. GraBois had worked with the Manhattan boy’s parents, interviewed police investigators and developed a theory: The boy, who had gone missing while on the way to school one morning in 1979, had been abducted and killed by a convicted pedophile named Jose Ramos.

It was not an outlandish notion. Ramos, it turned out, had been a onetime boyfriend of Patz’s regular babysitter. He could well have known the boy’s route to school. And he had a record of abusing young children.

GraBois, as part of his pursuit, came to work with an informant in the New York prison where Ramos was an inmate. Get him to tell you what he did to the boy, GraBois told the informant. Figure out what he had done with the boy’s body, which had never been found.

And so the informant, a man named Jack Colbert who was imprisoned on a fraud conviction, soon got to work at the federal facility in Otisville, N.Y. He says he first met Ramos in the prison law library, and that Ramos soon asked him for help appealing his sex crime conviction. Sometimes the two attended Jewish services together. They eventually came to share a cell for a couple of months. Ramos, the informant says, occasionally would talk about the Patz case.

One night, Ramos woke him in a panic. Colbert told ProPublica what happened next.

“There’s no body,” Ramos, wild-eyed, said. “If there’s no body, can they convict me?”

In the coming weeks, Colbert may get the chance to tell his story to a jury in the murder case against Patz’s accused killer. The accused, however, is not Ramos, but instead a former bodega clerk from Patz’s Manhattan neighborhood who, out of the blue in 2012, confessed to strangling the boy on the morning he went missing in May 1979. The former clerk, Pedro Hernandez, has since recanted the confession, and his lawyer has argued that he is a mentally ill man who was manipulated into a confession by detectives eager to solve one of New York’s most notorious missing child cases.

The lawyer, Harvey Fishbein, has made clear that he intends to argue to the jury that it is much more likely that Ramos killed Patz. Ramos, he has noted, had access to the boy and a history of abuse.

GraBois is expected to testify as part of Hernandez’s defense. And Colbert, whose account has only appeared in a 2009 book about the Patz case, may testify, as well. The jury has been publicly notified that Colbert may be called as a witness.

Colbert’s name was not disclosed in the 2009 book, “After Etan: The Missing Child Case that held America Captive,” by journalist Lisa Cohen. In 2013, ProPublica learned the informant’s identity, and interviewed him as part of its examination of the case against Hernandez. Colbert’s account of his time with Ramos was consistent with the version in the book and the one he had given GraBois years ago. GraBois has used Colbert’s information as he tried over the years to get New York prosecutors to charge and try Ramos.

Fishbein has said he intends to have Ramos himself testify at Hernandez’s trial, although the judge in the case has so far indicated he will not allow that to happen. At a hearing in 2013, Ramos refused to answer questions from reporters about any role he might have played in the Patz case, according to a local news account. Telephone calls to his lawyer were not returned.

When ProPublica spoke to Colbert in 2013, he lived in a dilapidated home in the Bronx brimming with canned goods and boxes upon boxes of documents. During the interview, he sat hunched over an antiquated desktop computer. He took a call about an unnamed business deal involving alcohol and buyers in China.

He was physically disheveled, as well. He wore a dirty t-shirt and shorts that exposed a sore on his knee the size of a man’s fist, oozing puss. He was scattered and eccentric and went on long conversational tangents. He was once interrupted by a friend in the house who chimed in with his own prison story. He rarely answered a question directly.

But, with time, Colbert went over the details of his relationship with Ramos. He described how he met Ramos, how he was recruited to spy on him, and how he gained the information he and the Patz family once hoped would land Ramos behind bars for life. While he said he still feared Ramos, he was no less sure of Ramos’ guilt.

“I believe he did it and I believe he did other kids too,” Colbert said.

In 1989, Colbert was three years into a 13-year prison sentence. He had been convicted of running an international fraud scheme – purchasing expired chemicals in the United States and selling them at a large profit in developing nations.

Ramos, Colbert said, cut an odd figure at Otisville. He was unkempt. He stank. He attempted to grow and wear his hair in the style customary for Hasidic Jewish men. Ramos, in fact, was Puerto Rican and from the mostly Catholic South Bronx.

Colbert said Ramos soon told him he knew the Patz family personally because a woman he described as his “wife” used to babysit Etan. Ramos said he’d been in the Patzs’ SoHo apartment, and he described its high ceilings.

Colbert, it turned out, knew GraBois. He had met him once at the U.S. Attorney’s office in New York when he was first brought in on the fraud charges. They were from the same Bronx neighborhood. So, in 1990, the informant wrote to GraBois, relayed what Ramos had told him, and offered further help.

By then, GraBois was eager to make a case against Ramos. He had questioned him once, two years earlier. Ramos had told him he saw Etan in Washington Square Park the day he went missing, that he had brought him back to his apartment, and that he had tried to have sex with him. The boy had resisted, and Ramos said he put him on the subway.

GraBois arranged for Colbert to share a cell with Ramos in the prison’s Segregated Housing Unit. The two would spend nearly every waking minute together. Colbert said he wasn’t thrilled with the idea. He told ProPublica that Ramos was temperamental and possessive. When an elderly inmate once tried to use Ramos’ favorite typewriter in the law library, Ramos picked it up over his head and threatened to bludgeon the man with it.

Ultimately, Colbert had two separate stays with Ramos, once for two weeks, the second time for at least a month. Through both stints, Ramos would often ramble on about Patz, contradicting himself frequently. One minute, Colbert said, Ramos would say Patz was alive; the next, he’d say the boy was dead.

Colbert, by his account, managed to gather what felt like evidence. At one point, he and Ramos had a conversation about lower Manhattan and Ramos told him he knew where all of Etan’s school bus stops were located. He drew him a map.

GraBois eventually sent a second informant in to work on Ramos, and used the information provided by both men as part of an appeal to then Manhattan District Attorney Robert Morgenthau. For years, the District Attorney’s office had kept the case open. GraBois laid out his information: Ramos’ near confession to him; his known relation to the Patz babysitter; his history of sexual abuse; and the work of his two informants.

In 2002, Etan’s father, Stan Patz, met with Morgenthau’s deputies and begged them to prosecute Ramos.

“I just want to get the bastard who fucked over my son!” Patz, in a final plea, screamed at the Manhattan prosecutors, according to Cohen’s book, “After Etan.”

Morgenthau chose not to prosecute and eventually explained his decision.

“We spent a huge amount of time on that case,” he said. “If we could go to a grand jury, we would in a minute. There’s no sufficient evidence and there’s absolutely no reason to open a grand jury investigation where you don’t have any admissible evidence.”

GraBois was disappointed.

“I’ve always maintained that reasonable people differed,” GraBois told ProPublica in an interview in 2013. “They felt we didn’t have enough evidence—fine. We always felt we did. But it was their decision and I have to respect it.”

The Patz family did pursue a measure of justice by filing and winning a civil wrongful death case against Ramos in April 2004.

When Cyrus Vance succeeded Morgenthau in 2010, he too, declined a request to prosecute Ramos.

Now Vance’s office will have to reckon with the evidence against Ramos in a different way.