By Scott Malone

BOSTON (Reuters) - When they admitted Dzhokhar Tsarnaev's role in the Boston Marathon bombing, his lawyers acknowledged that he will almost certainly spend the rest of his life behind bars. But the admission may help spare him a death sentence, legal experts said.

Tsarnaev's defense team opened his trial this week by bluntly admitting that the 21-year-old defendant and his older brother planted the twin bombs that killed three people and injured 264 on April 15, 2013, and three days later fatally shot a police officer as they tried to flee the city.

In the testimony that followed, the jury heard survivors' graphic and sometimes tearful accounts of being thrown through the air by the bombs, seeing their legs blown away and one father's account of having to leave his 8-year-old son, who he knew would not survive, in order to save his younger daughter, who lost a leg but not her life.

That testimony is all part of the first phase of the trial in U.S. District Court in Boston, where the jury will determine if Tsarnaev is guilty. Only after that will it move to the question of whether to sentence him to death or life in prison without the possibility of parole.

"The defense idea is that the jury is going to get to take its anger out on Tsarnaev by finding him guilty and then, hopefully, they will be able to set that aside and determine the question of punishment, should be live or die, a little more neutrally," said John Blume, a law professor at Cornell University who heads the school's Death Penalty Project.

Prosecutors have already shown videos of Dzhokhar Tsarnaev and his 26-year-old brother, Tamerlan, walking through crowds at the race's finish line and heard from Jeff Bauman, who testified that he saw the older brother leave a backpack shortly before the blast tore his legs off.

Tamerlan Tsarnaev died following a gunbattle with police when the two tried to flee Boston three days after the attack and Dzhokhar was found the next day, hiding in a drydocked boat where he had written a message describing the bombing as revenge for U.S. military action in Muslim-dominated countries.

Given the large amount of evidence against Tsarnaev, his lawyers are likely hoping that admitting guilt will help them win the jury's trust.

"If you know from a trial strategy perspective that the government is going to be able to prove the case, you don't want to lose credibility with the jury by standing there and saying, 'My guy didn't do it,'" said Walter Prince, a Boston attorney in private practice and former federal prosecutor.

Defense attorneys have passed on cross-examining any of the victims or first responders who testified in the trial's first two days, leaving the prosecution to make its case and moving the guilt phase of the trial along more quickly than expected.


The fact that Tsarnaev has not pleaded guilty suggests that federal prosecutors were not willing to back down from their efforts to have him sentenced to death, legal experts said.

Defense attorney Judith Clarke in her Wednesday opening statement laid out her argument for sparing Tsarnaev's life, that he was simply following the lead of a domineering older brother and not the driving force behind the attack.

U.S. District Judge George O'Toole cut her off at one point, noting that he would not allow "much" discussion of the pair's relative blame until the jury rules on Tsarnaev's guilt.

O'Toole allowed testimony by some bombing survivors about the repeated surgeries they underwent to repair legs ripped apart by shrapnel, despite complaints by the defense out of the jury's hearing that such testimony was better suited to the sentencing phase of the trial.

But he drew a line when two Boston Police officers who were U.S. military veterans drew comparisons between the bombing and their service in Iraq.

"I don't want to hear the word 'Iraq' or 'Afghanistan' without prior permission until we get to that other stage of the case," O'Toole said on Thursday. The court recessed on Friday with testimony set to resume Monday.

Despite O'Toole's efforts to keep the early part of the trial focused on guilt, both sides will be working to bolster their arguments for and against a death sentence, legal experts said.

"What they're doing is pre-conditioning the jurors," said David Weinstein, a Florida attorney who in prior jobs as a state and federal prosecutor brought death-penalty cases. "It's virtually impossible for a juror to sit through the guilt phase and not have an impression made in their mind about what's going to happen in the penalty phase."

(Reporting by Scott Malone; Editing by Tom Brown)