Tamerlan Tsarnaev, 26, died four days after the April 15, 2013 attack that killed three people and injured 264. His younger brother, Dzhokhar Tsarnaev, last month was convicted of carrying out the attack and could be sentenced to death.
Defence lawyers on Tuesday (local time) continued to call witnesses as they made their case that the surviving brother, 21, should be sentenced to life in prison without possibility of release rather than death for carrying out one of the highest-profile attacks on U.S. soil since September 11, 2001.
The lawyers, who at the trial's opening in March conceded that Dzhokhar Tsarnaev had committed all the crimes of which he was accused, contend that Tamerlan was the driving force behind the bombing, with his younger brother coming along out of a sense of sibling loyalty.
Amanda Ransom, 25, told jurors on Tuesday that her college friendship with Katherine Russell, now Tamerlan's widow, unraveled as Russell's relationship with the older Tsarnaev brother progressed.
Ransom testified that Tamerlan was an outgoing, flashy dresser when he met Russell at a nightclub and the two began dating. But Ransom later became concerned after Tamerlan cheated on Russell, fought loudly with her and laughed after tricking her into believing she might have contracted AIDS from him.
"At one point I heard him laughing really hard, and she was crying," Ransom testified, referring to the AIDS incident. Tamerlan did not have AIDS, she said.
Russell, who had not been raised Muslim, began wearing an Islamic hijab as her relationship with Tamerlan became more serious and distanced herself from her friends, Ransom testified. Ransom said she ultimately moved out of the apartment she shared with Russell in the dead of the night after Tamerlan threatened her when she tried to intervene in a fight between the couple.
Prior witnesses called by the defense have testified that Dzhokhar Tsarnaev was very unlike his aggressive brother, growing from a happy child into an easygoing young man who liked Domino's pizza and rap music.
On Monday, the jury heard from Tsarnaev's Russian relatives, who testified that they had been distressed when his mother and Tamerlan began to espouse radical Islamic views after moving to the United States.
Federal prosecutors earlier presented evidence showing that Dzhokhar Tsarnaev's computers contained radical jihadist literature, including copies of al Qaeda's "Inspire" magazine, and noted that he left a note suggesting the attack was an act of retribution for U.S. military campaigns in Muslim-dominated countries.
Martin Richard, 8, Chinese exchange student Lu Lingzi, 23, and restaurant manager Krystle Campbell, 29, died in the bombing. The Tsarnaev brothers shot dead Massachusetts Institute of Technology police officer Sean Collier three days later.
Elizabeth Zamparelli, a 22-year-old high school friend of Dzhokhar Tsarnaev, testified that she believed it was a joke when she heard her friend was a suspect in the Boston Marathon bombings: "It's very much not how my friends and I knew him to be."
