(New throughout with quotes from hearing)
By Scott Malone
BOSTON, April 16 (Reuters) - The accused Boston Marathon bomber's relationship with family, and particularly with his slain older brother who prosecutors contend helped plan the attack, is key to understanding his case, defense lawyers argued on Wednesday.
Attorneys for 20-year-old Dzhokhar Tsarnaev argued in federal court that the defendant should be able to meet with his sisters without being monitored by an FBI agent cooperating with prosecutors, and needs access to more information investigators have gathered on his dead brother to mount a full defense to charges that carry the threat of execution.
Tsarnaev is accused of planting two homemade bombs with his brother at the finish line of the April 15, 2013, Boston Marathon, killing three people and injuring 264 and, three days later, in the shooting death of a university police officer.
"Dzhokhar Tsarnaev is the youngest of three children. The eldest, Tamerlan, is dead and that leaves his two sisters," defense attorney David Bruck said in U.S. District Court in Boston. "The search for understanding what happened is in a large measure the story of this family and the relations between the people in it. It is our job to consider it and to learn about it."
Bruck argued that allowing an agent with the Federal Bureau of Investigation to monitor meetings between Tsarnaev, his sisters and attorneys, and pass the content of those meetings to prosecutors had limited the defendant's willingness to speak freely.
Legal experts have said that showing Tsarnaev as having normal relations with his family, and having been to some degree dominated by 26-year-old Tamerlan, could serve to reduce his blame in the largest mass-casualty attack on U.S. soil since Sept. 11, 2001. That, in turn, could increase his odds of avoiding execution if he is convicted, they have said.
District Judge George O'Toole said he was inclined to grant the request and urged prosecutors and defense attorneys to develop a plan to allow him to meet with family without having his comments passed to prosecutors.
"My interest in this question is not a broad one, but whether it could work in inhibiting the development of the defense case, and I think in this instance it does," O'Toole said.
Prosecutors also argued against the defense's request to see more of the government's files of a Chechen immigrant shot dead in Florida last year when he attacked agents investigating a 2011 triple murder in which the elder Tsarnaev was a suspect.
"Unless there is something in there that somehow relates to Dzhokhar Tsarnaev ... it seems to have no relevance," said Assistant U.S. Attorney William Weinreb.
O'Toole declined to rule on the matter immediately, asking to review the confession that FBI agents had been taking from Ibragim Todashev prior to the shooting, before he made a decision.
Tsarnaev, who is being held in a prison outside Boston, was not present in court on Wednesday. One of the people injured in the attack, Marc Fucarile, who lost his right leg, attended the conference but declined to speak with reporters afterward.
Ethnic Chechens, the Tsarnaev family immigrated to the United States about a decade before the attack. (Reporting by Scott Malone; Editing by Ellen Wulfhorst and Gunna Dickson)