A former FBI agent lied to jurors during mobster James "Whitey" Bulger's trial and overstated his professional accomplishments, including falsely claiming to be the first officer who recovered the rifle used to assassinate Martin Luther King Jr., federal officials said Thursday in announcing a perjury case against him.

Robert Fitzpatrick, who was once second in command of the Boston FBI division, surrendered to U.S. marshals with his lawyer after learning there was a warrant for his arrest.

Fitzpatrick, the first witness called by Bulger's attorneys during his 2013 racketeering trial, said he tried to persuade the FBI to terminate Bulger as an informant because the mobster didn't appear to be helping its mission to gather information on the Mafia. Fitzpatrick said his bosses did not act on his recommendation to close Bulger as an informant.

Prosecutors suggested during the trial that he exaggerated that claim to sell copies of a book he wrote about Bulger.

Fitzpatrick pleaded not guilty in federal court Thursday afternoon to six counts of perjury and six counts of obstruction of justice.

Prosecutors did not seek to hold him in custody while he awaits trial, agreeing with his lawyer that he could be released on a $50,000 unsecured bond.

"Mr. Fitzpatrick adamantly maintains his innocence and looks forward to challenging the government's accusations as soon as possible," his lawyer, Robert Goldstein, said after the brief hearing.

The 85-year-old Bulger is serving two life sentences after his 2013 racketeering conviction tying him to 11 murders and other gangland crimes in the 1970s and `80s.

During Bulger's trial, prosecutor Brian Kelly started his cross-examination of Fitzpatrick by asking him if he was a man who likes to make up stories. Fitzpatrick denied that.

Kelly went on to press Fitzpatrick about a claim he had made previously that he was the first officer at the scene who recovered the weapon used to kill King.

"Isn't it true that three Memphis police officers found the rifle that was used to kill Martin Luther King, not Bob Fitzpatrick?" Kelly asked.

"I found the rifle along with them. They could have been there ... but I'm the one that took the rifle," Fitzpatrick said.

Kelly then told Fitzpatrick that a report says someone else took the rifle from police officers and turned the bundle over to the FBI three hours later. "I took the bundle from the scene," Fitzpatrick explained.

Fitzpatrick also told jurors that in 1981, about six years after Bulger began working an informant, he was given the task of assessing whether the mobster was providing the FBI with useful information. The ex-agent insisted that he repeatedly sought to end the FBI's relationship with Bulger, particularly after Bulger became a suspect in two 1982 killings.

During the trial, prosecutors suggested that Fitzpatrick also exaggerated that claim.

The indictment alleges that since 1998, Fitzpatrick "has falsely held himself out as a whistleblower who tried to end the FBI's relationship with Bulger."

Bulger's lawyers argued during his trial that he was not an informant, and Fitzpatrick testified that Bulger told him he wasn't one. The indictment says Bulger never made that denial.

Hank Brennan, one of Bulger's lawyers, said he believed Fitzpatrick's testimony and suggested the ex-agent was prosecuted because he "stood up to the strength of the federal government and defied them."

Brennan added, "If you defy them, they'll crush you."

Fitzpatrick, 75, of Charlestown, Rhode Island, worked for the FBI from 1965 to 1986. In 1980, he was assigned as an assistant special agent-in-charge of the FBI's Boston division. In that position, he supervised the division's organized crime squad.

Prosecutors say Bulger was an informant for the squad from approximately 1975 through 1990.

The indictment says that in May 1986, Fitzpatrick was demoted and reassigned to the Providence, Rhode Island, field office. He left the FBI shortly after that, in December 1986.

On Thursday, Fitzpatrick did not speak during the hearing except to say "not guilty" when asked for his plea. The conditions of his release include that he give up his passport and limit his travel to within New England.

He is due back in court on June 10 for a status hearing.