Kevin Weeks testifies against his former mentor, James ‘Whitey’ Bulger

Kevin Weeks, the loyal tough guy whom James “Whitey” Bulger groomed as his protege, described in vivid detail Monday morning in US District Court in Boston how he first assisted Bulger in a murder.

It was May 1982, Weeks testified, and Bulger had been told that Brian Halloran — whom he’d been told was speaking to the FBI about Bulger — was at a payphone by the waterfront in South Boston. So Bulger assembled a hit team, and Bulger wore a fake mustache, which made him look like one of Halloran’s enemies, according to Weeks.

Testifying at Bulger’s racketeering trial, Weeks described for jurors how he conducted surveillance at Anthony’s Pier 4 restaurant, and reported when Halloran was leaving the restaurant. After Halloran got into a car driven by Michael Donahue, Bulger rolled up and started firing.

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“There was a lot of people there, they were diving, running around, they were screaming,” Weeks said.

Halloran got out of the car, but Bulger pulled his car around, and started firing again, Weeks told jurors.

“Jim Bulger just started shooting at him,” Weeks said. “Brian Halloran went down, and Jim Bulger just kept shooting. His body was bouncing on the ground.”

Donahue, a father of three who was not a target, was also killed.

Weeks said another man had been in the back seat of Bulger’s car, but that he wore a mask, and Weeks did not know who the second man was, but members of Donahue’s family said they don’t believe him.

“He knows exactly who was in the car with Whitey that killed my father and I think they’ve been protecting him since Day One. So I think Weeks is a fat liar if you ask me,” said Donahue’s son, Tom Donahue, outside the courthouse.

“It tears you apart, hearing the horrific story ... going through the details. It’s horrible listening to that, you know it shakes my whole entire family,” Tom Donahue said of listening to Week’s testimony about the day of the shooting.

Weeks said he later threw the guns used in the shooting into the water at Marina Bay, and it was then that he was part of the gang.

“I was involved in a double homicide, so I knew there was no getting out. I knew I was really in,” he said. Weeks also described how he and Bulger ran their criminal organization through extortion, including extorting ownership of a liquor store. “Jim Bulger thought it would be a good source of legitimate income,” he said.

Follow Globe coverage of the Bulger trial on’s live blog.

Weeks is one of the government’s key witnesses. He was one of Bulger’s closest associates for several decades. He started working with Bulger as an enforcer in the late 1970s and by the early 1980s was by his side during most of their criminal exploits.

After Bulger fled in 1995, Weeks was indicted for racketeering in 1999 and faced 20 years in prison. By then, he had also learned that Bulger was an FBI informant, and he agreed to cooperate with authorities. Weeks led investigators to an unmarked grave in January 2000 across from Florian Hall in Dorchester, where they uncovered the remains of three alleged victims of Bulger: Deborah Hussey, Arthur “Bucky” Barrett, and John McIntyre.

Weeks admitted to being an accessory to five murders and served five years in prison.

Bulger has denied killing Hussey and another woman, Debra Davis, who are among the 19 people he is accused of slaying.

Weeks has testified in previous court proceedings that he was present in January 1985 when Bulger strangled Hussey inside a South Boston home.

Weeks co-authored his autobiography titled “Brutal: The Untold Story of My Life Inside Whitey Bulger’s Irish Mob.’’ Weeks also co-wrote a book titled, “Where’s Whitey?” released in 2010.

He testified Monday that he agreed to sell his story as part of an agreement he reached in US bankruptcy court, with proceeds going to some of Bulger’s victims.

Bulger has pleaded not guilty to all charges and is being held without bail. Through his attorneys, Bulger has insisted he was never an informant for the FBI.

Earlier Monday, Weeks told jurors this morning that he developed a friendship with the gangster while working as a bouncer at a South Boston bar, Triple O’s, that Bulger frequented. Eventually Weeks began working for Bulger full time, driving around with him, running errands.

“Sometimes I’d beat somebody up,” he said.

He also said Bulger and his partner in crime, Stephen “The Rifleman” Flemmi, would often walk around parks in South Boston so they could speak freely without concern they were being recorded. Prosecutors used Weeks to introduce photos of Bulger as evidence in court Monday.