Joe Kebartas holds a sign reading "Death Penalty is Murder" outside the trial of accused Boston Marathon bomber Dzhokhar Tsarnaev in Boston, Massachusetts, March 4, 2015. [Photo/Agencies]

BOSTON - A lawyer for the accused Boston Marathon bomber said at the start of his trial that their client bore responsibility for the attacks that killed three people and injured 264 with a blunt admission: "It was him."

But Dzhokhar Tsarnaev was a secondary player in the April 15, 2013 bombings at the famous race and the fatal shooting days later of a police officer, defense attorney Judith Clarke said in her opening argument in US District Court in Boston. She indicated that the 21-year-old's older brother, Tamerlan, was the prime mover.

The defense strategy, which did not include a move to change Tsarnaev's not-guilty plea, appeared aimed at sparing Tsarnaev from the death penalty. If he is convicted, the jury will decide whether he is executed or gets life in prison without possibility of parole.

A prosecutor, William Weinreb, told jurors how Tsarnaev and his brother, both ethnic Chechens, carefully selected the places where they left the bombs in an effort to punish the United States for military actions in Muslim-dominated countries.

Assistant US Attorney Weinreb anticipated the defense's strategy of casting blame on 26-year-old Tamerlan Tsarnaev, who was killed when his younger brother inadvertently ran him over after a gunbattle with police a few days after the bombing.

"The focus is going to be on the defendant," the prosecutor said. "That's because it's his day in court. He's the one the government has to prove guilty, not his brother."

But Clarke said although Dzhokhar Tsarnaev, then 19, set off a homemade pressure-cooker bomb at the marathon's crowded finish line and three days later participated in the fatal shooting of the police officer did not tell the whole story.

"If the only question was whether that was Dzhokhar Tsarnaev ... it would be very easy for you,"

Clarke said her client played a secondary role in a scheme hatched and driven by his older brother.

"It was Tamerlan Tsarnaev who self-radicalized. It was Dzhokhar who followed him," Clarke said. "The evidence will show that Tamerlan planned and orchestrated and enlisted his brother into this series of horrific acts."

The approach set up an immediate conflict with US District Judge George O'Toole. He ruled shortly before opening statements that the question of the relative culpability of the two brothers was best left to the trial's second phase, which would follow providing Dzhokhar is found guilty.

"Some evidence of the brother's interactions will be inevitable," O'Toole allowed in brief remarks before the trial opened. But he interrupted Clarke several times to warn her against going too deep into family history so early in the trial.