Boston (CNN) -- Jurors squirmed Monday as a Massachusetts pathologist detailed the wounds inflicted on the 19 bodies prosecutors have lain at the feet of reputed Boston mob boss James "Whitey" Bulger.
Richard John Evans, the state's former chief medical examiner, described gunshot wounds to the temple, neck, spinal cord or heart in graphic detail. The jury in Bulger's federal racketeering and murder trial was both riveted and visibly uneasy during Evans' testimony, with some of them holding their hands over their mouths as he outlined the damage inflicted by bullets in 17 of the 19 cases.
Others twisted in their chairs during the testimony, while many intently took notes. But about eight victims into the litany of death certificates, the jurors appeared more relaxed.
Most of the victims were shot repeatedly in the head or neck, Evans testified. They included William O'Brien, who was expecting a baby boy when he was shot 20 times on a Boston boulevard in 1973.
"The most significant of the 20 was a wound to his right shoulder which pierced the spinal cord and lodged in his neck region," Evans explained. "It severed the spinal cord five centimeters from the bottom of the brain stem, which would make breathing impossible."
Former gang associate Francis "Buddy" Leonard was found in 1975 riddled with 13 bullets, including two to the left side of the head and one in the neck, according to his death certificate.
Another victim, Brian Halloran, was shot 14 times in 1975. All of the shots were "through-and-throughs," leaving no with bullets passing completely through the body, Evans testified.
Dead alongside him was Michael Donahue, Evans testified, a friend who wasn't affiliated with Boston's criminal gangs. Donahue was giving Halloran a ride home from a bar; his cause of death was "a gunshot wound to back of head that went into his brain," Evans explained.
Donahue's son, Tommy Donahue, sat with his head in his hands and his eyes closed during the description of the shot that killed his father.
"Going through it was rough, but I know every word on that death certificate," Donahue said after Monday's testimony. He brandished the document, which he has carried to court every day since the beginning of the trial, as he spoke.
Prosecutors say Bulger had a nearly 20-year reign of terror as the head of the Winter Hill Gang, the Irish mob that once terrorized South Boston. But during much of that time, they say, he was an FBI informant, and that rogue FBI agents tipped him off to his impending arrest and allowed him to flee a 1995 indictment.
Now 83, Bulger was captured in California in 2011 and brought back to Boston for trial, where a succession of mob figures have tied him to the killings.
Former FBI Agent Gerald Montanari testified Monday that Halloran was a "mid-level strong arm" who had agreed to wear a listening device and testify against Bulger and his associates. He was protected by the FBI and stationed in a safe house on Cape Cod until he began to "waffle" about entering into the witness protection program and refused a polygraph, Montanari said.
Montanari said the FBI cut ties with Halloran in 1982. The mobster was found shot to death shortly afterward, and earlier testimony stated that Bulger had Halloran killed after his FBI handler tipped him off to Halloran's cooperation.
Former hit man John Martorano -- now the government's star witness -- recounted Bulger's involvement in 13 killings, including O'Brien's and Leonard's. Former associate Kevin Weeks implicated Bulger in three more, including that of Halloran -- whose body was "bouncing off the ground" with every shot, he testified.
Prosecutors expect testimony later this week from Bulger's former partner and fellow informant Stephen "The Rifleman" Flemmi to lock up their case. Weeks, Flemmi and Martorano have all testified in exchange for reduced sentences in other murders.
CNN's Laura Batchelor and Deborah Feyerick contributed to this report.