Convicted Boston Marathon bomber's life is on the line as defense argues against execution

Lawyers for Boston Marathon bomber Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv's) are returning to federal court to make their case that he should be spared the death penalty.

Tsarnaev's defense team is set to begin presenting witnesses Monday in the penalty phase of his trial — the stage that will determine whether he is executed or spends the rest of his life behind bars.

Tsarnaev already has been convicted of 30 federal charges in the twin bombings that killed three spectators and injured more than 260 others on April 15, 2013. Seventeen of those charges carry the possibility of the death penalty.

Prosecutors are pushing for 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.