BOSTON — Boston Marathon bomber Dzhokhar Tsarnaev’s life is on the line as his lawyers return to federal court to make their case he should be spared the death penalty.

Tsarnaev’s defense team is set to begin presenting witnesses today 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, 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. Seventeen of those charges carry the possibility of death.

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 at the time of the attacks Tsarnaev was a 19-year-old college student flunking out of school and heavily influenced by his radicalized brother, Tamerlan, 26.

The witness list has not been made public, but legal experts expect the defense to call family members and friends.