A fiery federal prosecutor who has spent 20 years trying to end the alleged criminal career of James “Whitey” Bulger came near tears today as he defended a Massachusetts State Police detective whose integrity was under attack by lawyers for the South Boston gangster.

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Assistant US Attorney Fred Wyshak was unexpectedly emotional in federal court as Bulger’s lawyers accused State Police Lieutenant Steve Johnson of protecting convicted hitman John Martorano after a report that Martorano had returned to crime after being released from prison. Martorano is expected to be a key witness in Bulger’s trial.

With his voice cracking and tears forming in his eyes, Wyshak called Johnson “one of the best law enforcement officers I know, and [I’ve] worked with him for 20 years.’’

He said another state trooper did allege that Johnson was improperly protecting Martorano, but Wyshak said the claim was thoroughly investigated and dismissed as “unfounded.’’

Wyshak described the trooper who filed the report in unusually harsh terms, given that both are members of the law enforcement community. “The real culprit is a state trooper who went off the deep end and made allegations he could not support,’’ Wyshak said. He said the trooper recanted when questioned by investigators.

The dramatic moment came during the latest verbal exchange between prosecutors in US Attorney Carmen Ortiz’s office and attorneys for the 83-year-old Bulger, who is accused in a 32 count federal indictment based on investigative work by Johnson and US Drug Enforcement Administration Agent Dan Doherty.

The defense accusation ironically resembled one of the prosecution’s allegations against Bulger: that he was protected by corrupt FBI agents when he served as an informant for the agency, which helped him escape justice during a fearsome reign in Boston’s underworld in which he killed 19 people.

US District Court Judge Denise J. Casper, who began overseeing the jury selection process on Tuesday, indicated today that the prosection and defense had reviewed questionnaires for 500 jurors and 149 had been picked for further questioning on Monday. Another 385 questionnaires must still be reviewed. Eighteen jurors will ultimately listen to evidence during the trial, with 12 deliberating in the end.

Opening statements could come as early as next Wednesday in a long-awaited trial that is expected to last into September.

Aside from finding jurors, Casper must address a flurry of last-minute motions, including the one from the defense that questioned Johnson’s handling of Martorano.

Martorano has admitted to committing some 20 murders. He was sentenced to 12 years and was released from prison in 2007 under an agreement with prosecutors that required him to testify against Bulger allies. The agreement now requires him to take the stand against Bulger himself.

In court today, defense attorney Henry Brennan said prosecutors had only recently notified them of the complaint about Johnson and Martorano.

“It certainly goes to the credibility of the witness,’’ Brennan told Casper. “It’s not only to impeach Mr. Martorano; it’s to impeach his handlers.”

Casper took the request under advisement.

Bulger has pleaded not guilty to all charges and is being held without bail. Bulger has insisted he was not an FBI informant, while at the same time saying that he had been granted immunity for his crimes.

In other issues, Casper ruled today that veteran Boston Globe journalists Kevin Cullen and reporter Shelley Murphy can cover the Bulger trial, rejecting a request from Bulger that the two journalists be banned from the courtroom because his lawyers might call them to impeach prosecution witnesses.

Cullen, a columnist, and Murphy, a reporter, are the authors of the recently published book “Whitey Bulger: America’s Most Wanted Gangster and the Manhunt that Brought Him to Justice.’’ Cullen and Murphy have also chronicled Bulger’s criminal career for decades for the Globe.