The jury began deliberations at about 10:45 a.m. Tuesday after nearly two months of testimony, six hours of closing arguments and more than an hour and a half of instructions.
Judge Denise Casper dismissed the jury for the day at about 4:30 p.m. They will continue weighing the case Wednesday morning.
Bulger, 83, is charged with participating in 19 murders during the 1970s and '80s while he allegedly led the Winter Hill Gang.
During closing arguments Monday, Bulger's lawyers tried to put the government on trial, accusing federal prosecutors of making sweetheart deals with ruthless killers and pathological liars.
A prosecutor called Bulger "one of the most vicious, violent and calculating criminals ever to walk the streets of Boston."
Bulger was one of the FBI'S most wanted fugitives after he fled Boston in 1994 ahead of an indictment. He was finally apprehended in Santa Monica, Calif., in 2011.
Bulger's 16 years on the run embarrassed the FBI and exposed the bureau's corrupt relationship with its underworld informants.
During the trial, prosecutors said Bulger was a longtime FBI informant who ratted out members of the rival Mafia as well as members of his own gang. Bulger's lawyers strongly denied that, and said Bulger's former FBI handler, John Connolly, fabricated informant reports on Bulger to advance his own career at a time when bringing down the Mafia was a national priority.
Bulger's lawyers attacked the credibility of three key government witnesses: former hit man John Martorano, one-time Bulger protege Kevin Weeks and mobster Stephen "The Rifleman" Flemmi.
Defense attorney J.W. Carney Jr. argued that their testimony was bought and paid for by prosecutors.
Martorano and Weeks have completed their prison sentences and are free. Flemmi struck a deal to avoid the death penalty and is serving a life sentence.
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