BOSTON — A lawyer for Boston Marathon bomber Dzhokhar Tsarnaev urged a jury Monday to spare his client’s life, saying there is no punishment that he can get that would be equal to the suffering of the victims.

Lawyer David Bruck addressed the jury in the penalty phase of Tsarnaev’s federal trial. This stage will determine whether he is executed or spends the rest of his life behind bars.

“There is no evening the scales,” Bruck said in his opening statement. “There is no point in trying to hurt him as he hurt because it can’t be done.”

Tsarnaev, 21, already has been convicted of 30 federal charges in the twin bombings that killed three spectators and injured more than 260 others near the marathon’s finish line on April 15, 2013. He was also convicted of killing Massachusetts Institute of Technology police Officer Sean Collier days after the bombings. Seventeen of those charges carry the possibility of the death penalty.

Prosecutors are pushing for his execution. Their case in this second phase of the trial lasted just three days. Tsarnaev’s defense is expected to take up to two weeks.

His lawyers’ primary task will be to humanize Tsarnaev and undermine prosecutors’ depiction of him as a ruthless and heartless terrorist who placed a bomb just feet from a group of children and targeted the marathon for maximum bloodshed.

In the trial’s first phase, the defense tried to show that at the time of the attacks, Tsarnaev was a 19-year-old college student flunking out of school and heavily influenced by his radicalized older brother, Tamerlan, 26.

The defense likely is to continue emphasizing that theme but may also focus on Tsarnaev’s seeming aimlessness to show that he did not appear to be motivated by political concerns and that his late brother was the driving force behind the attack, aimed at punishing the U.S. for its wars in Muslim countries.