Federal prosecutor William Hogan Jr. on Friday had to relive the cases that made his career, and also nearly ended it, taking the witness stand in a wrongful conviction lawsuit brought by a former high-ranking member of the El Rukn street gang.

Nathson Fields was one of top targets in the criminal prosecutions that dismantled the El Rukn’s criminal empire on the city’s South Side during the early 1990s, and a string of victories in federal court all but dismantled the Rukns’ South Side criminal empire. As the lead prosecutor on those cases, Hogan had been a rising star, until allegations that he covered up for jailhouse misconduct by key witnesses nearly cost him his career.

Fields landed on death row after twice being convicted of a 1984 double-murder; his first conviction was thrown out when the presiding judge, Thomas J. Maloney, was found to have taken, then reneged on, a $10,000 bribe to fix the case. Fields claims a trio of Chicago detectives hid evidence that would have exonerated him; he was released after the CPD in 2010 located a “street file” in a dingy cabinet in a South Side police station that pointed to other suspects in the killings.

Fields also is getting a second trial in his civil lawsuit against the city. A federal jury earlier this year awarded Fields $80,000 in his wrongful conviction suit, but U.S. District Judge Matthew Kennelly said jurors should have known that Earl Hawkins — a former El Rukn hitman and the star witness in Fields’ murder trial decades ago — would gain early release from prison shortly after he testified in Fields’ civil case.

Hogan and two of the three detectives involved in Fields’ case backed Hawkins’ bid for parole, and Fields’ attorneys have said Hogan acted as Hawkins “personal representative” in parole proceedings.

On the witness stand Friday, Hogan scowled often as he sparred with Jon Loevy, Fields’ lawyer. Loevy quizzed Hogan about decades-old grand jury documents and allegations that Hogan covered up the fact that star witnesses for the prosecution had flunked drug tests and had illicit conjugal visits while waiting to testify. Hogan seemed especially peevish during a line of questioning by Loevy about Kennelly’s ruling on Hawkins’ early release, and how it cast doubts on Hawkins’ credibility as a witness.

Hogan said he hadn’t heard about those credibility issues until recently.

“Two weeks ago was the first time you ever heard about this little problem?” Loevy asked.

“I knew the government made a filing (in response). I never read it,” Hogan said.

Fields’ trial is set to end next week.