By Scott Malone

BOSTON (Reuters) - Lawyers for the Boston Marathon bombing suspect sparred with federal prosecutors on Monday over how early in the trial they may discuss his older brother's role in the attack, suggesting the defendant's life may well hang on that question.

Dzhokhar Tsarnaev, 21, is charged with killing three people and injuring 264 in the April 15, 2013, attack as well as shooting dead a university police officer as he and Tamerlan Tsarnaev, 26, prepared to flee the city three days later.

Tamerlan died that night following a gunbattle with police but defense attorneys described him as the driving force behind the largest mass-casualty attack on U.S. soil since Sept. 11, 2001. They said the jury must understand his role to make a fair decision on whether to sentence Dzhokhar to death or life in prison if he is convicted in a trial due to begin on Wednesday and last into June.

"The lead conspirator, the person who started all this and without whom the Boston Marathon bombing would never have occurred, the older brother, is dead ... That presents a problem for the government's request for the death penalty," defense attorney David Bruck told U.S. District Judge George O'Toole.

"The government says the motive is extremist, jihadist ideology," Bruck said as Tsarnaev sat quietly in court, dressed in a sport jacket and open-collared shirt. "That opens the door for us to respond that a large part of the motive may have been the defendant’s love for, admiration of, submissiveness to, his older brother."

In a federal death penalty case, a jury must first determine whether the defendant is guilty and then consider his sentence. Prosecutors argued that Tsarnaev's lawyers should not focus on the older brother until the trial's second phase.

"They want to advance the theory of the co-conspirator's culpability in the liability phase," said Assistant U.S. Attorney Aloke Chakravarty. "It's simply not relevant."

'GRUESOME AND GRAPHIC'

The two sides also argued about how much graphic evidence the jury will be shown during the trial, which will recount how bombs tore through a crowd of thousands of people at the race's crowded finish line, killing restaurant manager Krystle Campbell, 29; graduate student Lingzi Lu, 23, and Martin Richard, 8. Massachusetts Institute of Technology police officer Sean Collier, 27, was fatally shot three days later.

Defense attorneys asked that the jury not see full-body forensic photos of the victims, saying they would cause victims' families and jurors severe emotional distress.

"Full body is requested because they are injured everywhere on their bodies," said Assistant U.S. Attorney Nadine Pellegrini. "They are gruesome and graphic because they show the deaths of three young people by what we contend was a bomb."

The sides also argued about whether and how jurors might view the boat where Tsarnaev was found hiding at the end of a manhunt four days after the attack. Court documents say he left a note suggesting the attack was in retaliation for U.S. military campaigns in Muslim-dominated nations.

O'Toole did not immediately rule on the arguments.

Defense lawyers late Monday filed a fourth motion asking to move the trial out of Boston, saying that more than half of the remaining candidates from which the jury is set to be picked on Tuesday had a connection to the case or had already determined Tsarnaev to be guilty, based on their responses to questionnaires.

O'Toole three times rejected a similar request and an appellate panel on Friday also rejected it.

In a related case, the Council on American-Islamic relations said on Monday the parents of Ibragim Todashev, a friend of Tamerlan's shot and killed by an FBI agent during the investigation of the attack, plan to sue the federal law enforcement agency for $30 million.

(Editing by Bill Trott, Susan Heavey and Eric Walsh)