BOSTON — Dzhokhar Tsarnaev’s lawyers may try to save him from the death penalty in the Boston Marathon bombing by arguing he fell under the murderous influence of his older brother, legal experts say.The outlines of a possible defense came into focus this week when it was learned that Tsarnaev’s attorneys are trying to get access to investigative records implicating the now-dead brother in a grisly triple slaying committed in 2011.In court papers Monday, federal prosecutors acknowledged publicly for the first time that a friend of Tamerlan Tsarnaev told investigators that Tamerlan participated in the unsolved killings of three men who were found in a Waltham apartment with their throats slit, marijuana sprinkled over their bodies.The younger Tsarnaev’s lawyers argued in court papers that any evidence of Tamerlan’s involvement is “mitigating information” that is critical as they prepare Dzhokhar’s defense. They asked a judge to force prosecutors to turn over the records.Dzhokhar Tsarnaev, 20, faces 30 federal charges, including using a weapon of mass destruction, in the twin bombings April 15 that killed three people and injured more than 260. Tamerlan Tsarnaev, 26, died in a gunbattle with police days later.The government is still deciding whether to pursue the death penalty for the attack, which investigators say was retaliation for the U.S. wars in Muslim lands.Richard Dieter, executive director of the Death Penalty Information Center, said the defense may be trying to show that the older brother was the guiding force.“If I was a defense attorney and was seeking perhaps to draw attention to the influence the older brother had in planning the bombing, I would use his involvement in other crimes to show that he was likely the main perpetrator in the Boston bombing,” Dieter said.“I would take the position that my client, the younger brother, was strongly influenced by his older brother, and even if he is culpable, the death penalty is too extreme in this case.”Similarly, Aitan D. Goelman, who was part of the legal team that prosecuted Oklahoma City bombing figures Timothy McVeigh and Terry Nichols, said the defense may be looking to minimize the younger brother’s role in the bombing.“I think the most likely reason is that if they are arguing some kind of mitigation theory, that the older brother was a monster and the younger brother was under his sway or intimidated or dominated by him,” he said.Miriam Conrad, Tsarnaev’s public defender, had no comment.Investigators have given no motive for the 2011 slayings. One victim was a boxer and friend of Tamerlan Tsarnaev’s.Friends of those killed have said they gave Tsarnaev’s name to investigators at the time. That has raised questions of whether authorities missed an opportunity to prevent a bigger tragedy.Federal prosecutors said in court papers that Ibragim Todashev, another friend of Tamerlan’s, told authorities that Tamerlan took part in the killings. Todashev was shot to death in Florida by authorities while being questioned. Prosecutors argued that turning over the records would damage the investigation.