A one-time federal prosecutor testified as a defense witness at the Etan Patz murder trial Monday that a child molester named Jose Ramos once said he was "90 percent sure" he took the boy to his apartment for sex on the day Etan disappeared in SoHo in 1979.

The testimony from Stuart GraBois, a prosecutor in Manhattan in the late 1980s, is a linchpin in the effort of lawyers for defendant Pedro Hernandez to win his acquittal in the Supreme Court trial in Manhattan by casting suspicion on Ramos.

GraBois said he brought Ramos, who was imprisoned in Pennsylvania, in for questioning in 1988 because he had told Bronx prosecutors in 1982 that he had a relationship with a woman who walked Etan to school during a school bus strike in early 1979.

More storiesComplete coverage: Etan Patz case

Ramos, now 71, didn't know why he had been transferred to talk to federal authorities, GraBois said, but after some small talk, the prosecutor asked him, "How many times did you have sex with Etan Patz?"

"He froze," GraBois testified. "Then he said, 'I'll tell you all about it. I want to get it off my chest.' "

Ramos, GraBois said, related that after Etan's picture appeared on TV after his May 25, 1979, disappearance, Ramos thought he looked like a boy he had picked up near Washington Square Park that day.

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"I'm 90 percent sure this was the same boy I took back to my apartment," said Ramos, a street vendor who lived in a building on West Fourth Street and scavenged trash for a living, according to the ex-prosecutor.

Ramos claimed the boy resisted having sex, GraBois testified, so Ramos took him to an uptown subway to go to his aunt's house in Washington Heights. There has been no evidence at trial of any Patz family relative in that location.

At that point, GraBois said, Ramos asked for an attorney. He promised to repeat the story after talking to a lawyer, but never did. On the way out of their first meeting, GraBois testified, he and Ramos ran into then-U. S. Attorney Rudy Giuliani in the elevator.

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Ramos told Giuliani he "looked thinner" than he did on TV. When Giuliani was gone, GraBois recalled, Ramos turned to him and said, "You'll have Giuliani's job once I tell you everything."

Hernandez, 53, told police in 2012 that as a teen working in a SoHo bodega, he lured 6-year-old Etan into the basement and strangled him. The defense contends the confession was a fantasy brought on by a mental disorder.

Ramos' statements to GraBois have long been public, but Monday was the first time the ex-prosecutor recounted them in a criminal trial. He said he pushed new molestation charges in Pennsylvania to keep pressure on Ramos, and used two prison informants to elicit incriminating statements, but lacked federal jurisdiction and couldn't get New York officials to charge Ramos in the Patz case.

Prosecutors contend the case against Ramos is unconvincing. In cross-examination Monday, prosecutor Joan Illuzzi-Orbon portrayed GraBois as sloppy and over-aggressive, grilling him about missing records from his probe and about alienating the Patz family with rough questioning after he took over the case in 1985.

Ramos, who is still serving a Pennsylvania prison sentence, has said he will exercise his Fifth Amendment rights to not testify. Prosecutors have refused to grant him immunity so he could be forced to testify. Cross-examination of GraBois is scheduled to continue on Tuesday.