The lead prosecutor at the 2005 trial of John Giuca in the murder of college football player Mark Fisher took the stand Monday, insisting she had no clue her jailhouse-snitch witness, who would later recant his testimony, was untrustworthy.

Anna-Sigga Nicolazzi, now chief of trials for the Brooklyn DA, told the packed Brooklyn Supreme Court that she didn’t become aware of John Avitto’s mental illness — including schizophrenia and bipolar disorder — until the DA’s Conviction Review Unit met last year.

At Giuca’s hearing to determine whether he’ll get a retrial, Avitto last week recanted his testimony that Giuca had confessed to the murder, stating he “made up” evidence in exchange for help in quashing a warrant.

A poised Nicolazzi insisted that Avitto’s case manager did not tell her he was in and out of court for drug violations just days before he testified against Giuca.

She also maintained she hadn’t read prison psychology records that showed Avitto admitted he’d lied about being suicidal in prison in order to receive special attention, and was having hallucinations in the months before he was set to testify against Giuca.

“It says here he suffered visual hallucinations — something about snakes,” Giuca attorney Mark Bederow told the court.

“Do you think that would have been helpful for Mr. Giuca to know?” Bederow asked Nicolazzi.

“Yes,” she said.

Bederow also read from records that Avitto told a prison psychologist in April 2005, “I had to do what I had to do to change my situation.”

“Do you believe that would have been pretty compelling for the defense to discredit him?” Bederow asked.

“Maybe,” she said, adding, “When he testified in 2005, we had no doubts he was credible.”