BOSTON — A former FBI supervisor who wrote a book on reputed gangster James “Whitey” Bulger testified Monday that he tried to terminate him as an FBI informant, but his bosses would not do it.

Robert Fitzpatrick, a former assistant special agent in charge of the FBI’s Boston office, said that in 1981 he was given the task of assessing Bulger to see if he was providing the FBI with useful information on the Mafia. At that point, Bulger had been an FBI informant for about six years.

Fitzpatrick said that when he met with Bulger, he repeatedly changed the subject, “played the tough guy,” and said he would never testify against anyone.

“At one point, he even said he was not an informant,” Fitzpatrick said.

“Basically, he was not giving me any information that I was out there trying to get,” he said.

Bulger, 83, is on trial in a massive racketeering indictment charging him with playing a role in 19 killings while he allegedly led the Winter Hill Gang in the 1970s and ’80s. He was one of the FBI’s most wanted fugitives after he fled Boston in 1994 until he was captured in 2011 in Santa Monica, Calif. Bulger has strongly denied being an informant.

After his initial meeting, Fitzpatrick said, he recommended that Bulger be terminated as an FBI informant. But he said his supervisor and top brass at FBI headquarters in Washington did not act on his recommendation.

The defense has tried to show Bulger wasn’t an informant in order to undercut the credibility of prosecution witnesses. They claim his FBI handler, John Connolly, fabricated Bulger’s FBI file to advance his own career at a time when bringing down the Mafia was a national priority for the FBI. Fitzpatrick testified that he revealed misconduct by Connolly, who is now in prison.

Fitzpatrick, the first defense witness called after more than six weeks of prosecution witness testimony, said he was especially worried about the FBI’s relationship with Bulger in the case of Edward “Brian” Halloran, a man who had provided the FBI with information about Bulger’s alleged involvement in a 1981 killing.

Fitzpatrick said he met with a federal prosecutor in an attempt to get Halloran into the witness protection program. Halloran was gunned down two days later along with a man driving him home— killings prosecutors have accused Bulger of committing.

During cross-examination, Assistant U.S. Attorney Brian Kelly repeatedly asked Fitzpatrick if he is “a man who likes to make up stories” and suggested he falsely claimed credit for having a key role in several high-profile cases, some contained in his book on Bulger, “Betrayal.”

Kelly asked Fitzpatrick about claims he found the gun used to kill Martin Luther King Jr. in 1968 and passages from his book in which he says he arrested Boston Mafia underboss Gennaro Angiulo in 1983.

Kelly read from FBI reports that Memphis police had found the gun and other agents made the Angiulo arrest.

Fitzpatrick stood his ground, saying he’d been with the officers who found the rifle and was the supervisor of the agents who arrested Angiulo.

Kelly also suggested that Fitzpatrick implied in his book that he helped find the body of John McIntyre, a Quincy fisherman allegedly killed by Bulger.

Kelly said one photo in the book shows McIntyre standing in a field wearing a trenchcoat, on the other side of the page is a photo of investigators removing McIntyre’s body from a burial site in Boston in 2000.

Fitzpatrick acknowledged that he was not present when McIntyre’s body was unearthed.

Fitzpatrick will be back on the witness stand Tuesday.