By Scott Malone

BOSTON (Reuters) - The two youngest people killed by the Boston Marathon bombing were torn apart by one of the blasts that ripped through the crowd at the finish line, medical examiners testified on Monday as prosecutors wound up their case against accused bomber Dzhokhar Tsarnaev and defense attorneys began calling witnesses.

Tsarnaev faces death or life in prison if convicted. It is not clear if Tsarnaev will testify in his own defense.

Massachusetts' Chief Medical Examiner Henry Nields showed the jury 8-year-old Martin Richard’s bloodstained gray New England Patriots T-shirt with holes that correlated with injuries to the child’s torso.

On the 15th day of testimony in Tsarnaev's trial at Boston federal court, Nields said a piece of shrapnel appeared to have gone straight through Richard's body. "It would be difficult to say the precise location due to the size of the openings in the abdomen," he testified. Nields was the 92nd and final witness called by the prosecution.

Shrapnel from the same homemade pressure cooker bomb that killed Richard punched through Chinese exchange student Lingzi Lu's legs, causing the 23-year old to bleed to death within minutes, Boston medical examiner Katherine Lindstrom testified.

"Her injuries were caused by debris hitting her body and going through her body," Lindstrom said. "They would have been very painful."

Tsarnaev, 21, is charged with the April 15, 2013, bombing that killed three and injured 264 people, and the fatal shooting of Massachusetts Institute of Technology police officer Sean Collier three days later as he and his brother prepared to flee the city.

The brother, Tamerlan Tsarnaev, 26, died in the early morning hours of April 19 after a gunfight with police that ended when Dzhokhar inadvertently ran him over with a hijacked car.

Prosecutors have said Tsarnaev left the bomb that killed Richard and Lu in front of The Forum restaurant.

The third person to die, 29-year-old restaurant manager Krystle Campbell, was killed by another bomb, prosecutors said.


Defense lawyers opened the trial earlier this month with a blunt admission that Tsarnaev had done everything of which he had been accused by federal prosecutors. But they contended that he had done so out of a sense of subservience to his older brother rather than his own anger at his adopted country.

By painting Tamerlan as the driving force behind the attacks, the defense hopes to spare the younger Tsarnaev a death sentence and persuade the jury to determine that he should spend the rest of his life in prison.

The defense on Monday tried to poke holes in prosecution evidence about store purchases of pressure cookers and metal BB pellets used to make the bombs.

Computer forensics specialist Gerald Grant testified that at the time prosecution witnesses have shown those purchases were made north of Boston, electronic records suggested that the defendant was about 55 miles (89 km) south of Boston, around the University of Massachusetts at Dartmouth where he was a student.

Records showed that Tsarnaev's cell phone was located near the school at the time of both shopping trips in early 2012. A card associated with Tsarnaev was used to purchase a meal at a Wendy's in the vicinity of the school at the time the pressure cookers were purchased, said Grant, who works for the federal public defender's office in western New York state.

Under cross-examination by the prosecution, Grant acknowledged that the data indicated only what cell tower the phone had connected to at the time and did not show who had been using it.

The Tsarnaevs came to the United States from Chechnya about a decade before the attack, settling just outside Boston in Cambridge, Massachusetts.

Defense lawyers will be limited in how much evidence they put forward about the relative blame of the two brothers before the jury decides Tsarnaev's fate.

If the jury finds him guilty, the trial will enter a penalty phase, when both sides will call another round of witnesses before the same jury determines whether Tsarnaev should be sentenced to death or life in prison without the possibility of parole.

(Reporting by Scott Malone; Editing by Lisa Von Ahnm, Toni Reinhold and Cynthia Osterman)