BOSTON — Lawyers for reputed gangster James "Whitey" Bulger asked a judge Monday to delay opening statements in his racketeering trial so they can investigate a claim that law enforcement protected a hit man who is expected to be a star prosecution witness.

Bulger attorney J.W. Carney Jr. accused prosecutors of engaging in a "cover-up" of an allegation that John Martorano, who admitted killing 20 people, has committed crimes since his 2007 release from federal prison and has been shielded from investigation. Carney asked prosecutors to turn over all documents related to the allegation.

Carney said the allegation was made by a 20-year veteran of the state police who complained in an anonymous letter that his attempts to investigate Martorano were "stymied," in particular by state police Lt. Steve Johnson, one of the lead investigators in the Bulger case.

"The only thing that this can be called is a cover-up," Carney said.

Prosecutors opposed the delay, saying that the allegation was "thoroughly debunked" after an internal investigation by state police. They said the complaint was made by a disgruntled state trooper who falsely accused another trooper of ignoring allegations against Martorano.

"Mr. Martorano will not testify in any fashion that he was told or promised by any member of the government team ... that he could commit crimes and that the government would cover it up," said Assistant U.S. Attorney Fred Wyshak.

"That's fantasy, it's a fiction, it did not happen," he said.

U.S. District Judge Denise Casper did not immediately rule on the motion. The judge had said she hoped to have opening statements Wednesday.

The motion was filed as Casper began questioning potential jurors individually during jury selection for Bulger's trial. The 83-year-old former leader of the Winter Hill Gang is accused of a long list of crimes, including participating in 19 killings.

Several potential jurors were excused after they said they had read books or media accounts about Bulger's alleged crimes and had already formed an opinion that he is likely guilty. Several others were excused after they said it would be a financial hardship for them to serve on the trial, which is expected to last three to four months. Still others were excused after they told the judge they had booked a summer vacation.

Bulger fled Boston in 1994 and was one of the FBI's most wanted fugitives until he was captured in Santa Monica, Calif., in 2011.

Authorities say he was a longtime FBI informant who provided information on the New England Mafia, his gang's main rivals. Bulger has pleaded innocent and denies being an informant.