Lawyers for Boston Marathon bomber Dzkokhar Tsarnaev rested their case in his federal death penalty trial Tuesday after presenting a brief case aimed at showing his late older brother was the mastermind of the 2013 terror attack.

The defense admitted during opening statements that Tsarnaev participated in the bombings. But Tsarnaev's lawyer said he was a troubled 19-year-old who had fallen under the influence of his radicalized brother, Tamerlan, 26.

Closing arguments will be held April 6.

The defense has made it clear from the beginning of the trial that its strategy is not to win an acquittal for Tsarnaev but to save him from the death penalty.

If the jury convicts Tsarnaev — an event that seems a foregone conclusion because of his admitted guilt — the same jury will be asked to decide whether he should be executed or spend the rest of his life in prison.

During its brief case, the defense called a cell site analyst who showed that Tsarnaev was at the University of Massachusetts-Dartmouth when Tamerlan purchased components of the two bombs used in the 2013 attack, including pressure cookers and BBs.

Tsarnaev's lawyer told jurors that it was Tamerlan who shot and killed MIT police Officer Sean Collier three days after the bombings. Tamerlan died after a gun battle with police hours after Collier's slaying.

Prosecutors called more than 90 witnesses over 15 days of testimony, including bombing survivors who described losing limbs in the attack. Three people were killed and more than 260 were wounded when two pressure cooker bombs exploded near the marathon finish line on April 15, 2013.

Jurors saw gruesome autopsy photos of the three killed: 8-year-old Martin Richard, a Boston boy who had gone to the marathon with his family that day; 23-year-old Lingzi Lu, a graduate student from China who was studying at Boston University; and 29-year-old Krystle Campbell, a 29-year-old restaurant manager.