James 'Whitey' Bulger and his protege fight in court

James "Whitey" Bulger and his once-loyal criminal apprentice had an angry, profanity-laced exchange in court Thursday during Bulger's racketeering trial after the former aide called Bulger and his partner "the two biggest rats."

Tensions exploded as Kevin Weeks was questioned by Bulger's lawyer about a plea deal with prosecutors that resulted in Weeks serving five years in prison after pleading guilty to aiding and abetting Bulger in five murders.

Weeks bristled at attorney J.W. Carney Jr.'s suggestion that he had beaten the system and that he wasn't bothered by his participation in five killings.

"How does it bother you?" Carney asked.

"Because we killed people that were rats, and I had the two biggest rats right next to me," Weeks snapped.

"You suck," Bulger said from his seat at the defense table.

"F--- you, OK!" Weeks replied.

"F--- you, too," Bulger said.

"What do you want to do?" Weeks challenged Bulger.

Judge Denise Casper moved to restore order.

"Hey!" Casper said. "Mr. Bulger, let your attorneys speak for you. Mr. Weeks, here's how this works: You answer the questions, OK?"

Weeks was referring to the prosecution's claim that Bulger and his longtime cohort, Stephen "The Rifleman" Flemmi, were FBI informants on the rival New England Mafia while they were both committing a litany of crimes.

Weeks said he decided to cooperate with prosecutors and testify after learning that Bulger and Flemmi were informants.

The outburst came on the second day of Weeks' testimony against Bulger. Earlier Tuesday, he offered gruesome details about three murders he said he saw Bulger commit.

Bulger is accused of playing a role in 19 killings during the 1970s and '80s while he allegedly led the Winter Hill Gang.

Bulger has pleaded not guilty and insists he was never an FBI informant. During the trial, his lawyers have focused much of their energy on rebutting the informant claim.

Weeks, who started working for Bulger as an enforcer in the late 1970s, said Bulger was a mentor, friend and "like an older brother to me."

"He treated me great," he said.

Weeks said he helped Bulger even after Bulger fled Boston in 1994 when he learned he was about to be indicted.

That changed, Weeks said, when he learned that Bulger and Flemmi had been FBI informants for years. He said that went against the South Boston culture to never rat on your friends or your enemies.

Carney asked Weeks if he was concerned that he'd be seen as a rat after he made his deal with prosecutors to testify against Bulger and Flemmi.

"You can't rat on a rat," Weeks said.

Weeks described three killings he said he saw Bulger commit and several others he said Bulger admitted he had somehow engineered.

He said he was at a South Boston house with Bulger in 1985 when he heard that 26-year-old Deborah Hussey, the daughter of Flemmi's longtime girlfriend, would be coming over.

While Weeks was upstairs, he heard a thud coming from below him.

"Jim Bulger had her on the ground, choking her," he said.

Flemmi carried her to the basement but thought she was still alive. Flemmi wrapped a rope around her neck, put a stick in it and strangled her, Weeks said.

He said Flemmi then removed her teeth and they buried her under the dirt floor.

Hussey was one of three people who were buried in the same house after Bulger killed them, Weeks said. He said he helped moved the bodies later when the house was being sold. In 2000, after he began cooperating with the government, he led authorities to the new location in the Dorchester section of Boston.

Weeks said John McIntyre was killed after Bulger heard he may have spoken with authorities about a failed bid to send guns to the Irish Republican Army. He said McIntyre was chained to a chair and interrogated by Bulger, who held a machine gun.

After McIntyre admitted he was cooperating with law enforcement, Bugler wrapped a rope around his neck and tried to strangle McIntyre, Weeks said. The rope was too thick and the pressure caused McIntyre to vomit, he said.

"Jim says to him, 'Do you want one in the head?' and he said, 'Yes, please,'" Weeks said.

Bulger then shot him in the head, Weeks said. When that didn't kill him, Bulger shot him several more times, Weeks said.

During cross-examination, Bulger's lawyer focused largely not on the crimes Bulger is charged with, but instead on trying to get Weeks to acknowledge that Bulger loathed anyone who was a rat.

Carney asked Weeks if, during the two decades he spent working with Bulger, he made it clear that "what he hated above all else was informants."

"We killed people for being informants," Weeks said.

Carney also asked Weeks if Bulger was ever charged, despite being involved in "all manner of crimes," including extortion, loan-sharking, money-laundering and, sometimes "crimes of violence."

Weeks acknowledged that Bulger was not charged until 1995.

The defense contends that Bulger paid FBI agents, state police and local police for information on investigations so he could stay one step ahead of any indictment.

Weeks said he saw Bulger stuffing envelopes with cash at Christmas and Bulger told him the money was for various law enforcement officials, including six FBI agents he claimed he had corrupted.

Former FBI Agent John Connolly was convicted for tipping Bulger off to his 1995 indictment. Bulger fled in late 1994, just before the indictment, and was one of the nation's most wanted fugitives until he was captured in Santa Monica, Calif., in 2011.

Flemmi is serving a life sentence for 10 murders.