BOSTON - Prosecutors rested their case against Boston Marathon bomber Dzhokhar Tsarnaev on Monday after jurors in his federal death penalty trial saw gruesome autopsy photos and heard a medical examiner describe the devastating injuries suffered by an 8-year-old boy killed in the 2013 terror attack.

But Tsarnaev's lawyers began their defense by quickly trying to show that his older brother was the mastermind of the plan to detonate pressure-cooker bombs near the finish line of the famous race.

One of the first witnesses called by the defense was a data analyst who said Tsarnaev's cellphone was being used in southeastern Massachusetts - where he was attending college - while pressure cookers were being purchased north of Boston more than two months before the bombing. The analyst also testified that large quantities of BBs were purchased a little over a month before the attack in two Wal-Mart stores in New Hampshire, at a time when Tsarnaev's cellphone was again being used near UMass-Dartmouth.

The defense has made it clear from the first day of testimony on March 4 - when his lawyer admitted he participated in the bombings - that their strategy is not to win an acquittal but to save Tsarnaev from the death penalty by arguing that his brother, Tamerlan, was largely responsible for the bombings.

Prosecutors ended their case on an emotional note. At least three jurors cried and wiped their eyes with tissues as they looked at photos of 8-year-old Martin Richard, who went to watch the marathon with his parents and siblings on April 15, 2013, and was killed when the second of two bombs exploded near the finish line.

The boy's parents watched somberly from the second row of the courtroom. Bill Richard kept his arm around the shoulder of his wife, Denise, throughout the testimony.

Henry Nields, chief medical examiner for Massachusetts, said Martin received injuries to virtually every part of his body, including lacerations of his liver, left kidney and spleen, broken bones and third-degree burns.