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.

Dr. 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. His stomach was also ruptured.

Nields said he removed small nails, metal pellets, fragments of wood and black plastic from the boy’s wounds. He also displayed the blood-stained, shredded clothing that Martin was wearing when the bomb exploded.

Two other people were killed and more than 260 were injured in the bombings. Prosecutors believe the brothers were seeking retaliation against the U.S. for wars in Muslim countries.

The first defense witness was Michelle Gamble, an FBI field photographer who testified earlier Monday for prosecutors, describing various photos and a video showing the scene of the second blast both before and shortly after the explosions.

In one of the photos, Martin Richard, his sister and several other children stand on a metal barricade. Tsarnaev appears to be just a few feet behind Martin and his sister.

While cross-examining Gamble, Tsarnaev’s lawyers showed other photographs with several people in between Tsarnaev and the children, an apparent attempt to show that Tsarnaev didn’t purposefully target them with the bomb.

When the defense called Gamble as its first witness, Tsarnaev’s lawyer, Miriam Conrad, asked her about a book titled “Wiring” that was found during a search of the Tsarnaev family’s apartment in Cambridge. Gamble said the book was found under the living room couch.

Tsarnaev’s lawyers have tried to show that he was not living in the apartment when the bombings occurred because he was attending college. Tamerlan Tsarnaev, 26, was living in the apartment with his wife and their young daughter.

During their case, prosecutors presented heart-wrenching testimony from survivors who lost legs in the bombings. A string of first responders described a chaotic mix of smoke, blood and screams just after the bombs went off.

The defense will try to show that Tamerlan Tsarnaev was more culpable in the attack and in the killing three days later of Massachusetts Institute of Technology police Officer Sean Collier.

The defense case is expected to be relatively short. Once that is complete, jurors will deliberate on whether Tsarnaev is guilty of the 30 federal charges against him related to the bombing, the killing of Massachusetts Institute of Technology police Officer Sean Collier three days later and a violent confrontation with police in Watertown.

Tamerlan Tsarnaev was killed during the Watertown confrontation. Dzhokhar Tsarnaev, then 19, was found more than 18 hours later hiding in a boat parked in a yard.

If the jury convicts Tsarnaev — an event that may be a foregone conclusion because of his admitted guilt — the trial will move on to the second phase, when the same jury will hear more evidence to decide whether Tsarnaev should be put to death or should spend the rest of his life in prison.

During this second phase of the trial, Tsarnaev’s lawyers will present evidence of factors they believe mitigate his crimes, such as his age at the time and the influence of his older brother. The Tsarnaevs — ethnic Chechens — lived in the former Soviet republic of Kyrgyzstan and the volatile Dagestan region of Russia before moving to the U.S. with their parents and two sisters about a decade before the bombings.

Prosecutors will present evidence of aggravating factors, such as the brutality of the attack and the death of a child, to argue that Tsarnaev should be executed.