Metro

Bomb victim’s injuries described to Tsarnaev jurors

Tools and other items were found in the Cambridge apartment.
FEDERAL PUBLIC DEFENDER OFFICE
Tools and other items were found in the Cambridge apartment.

The testimony had grown repetitive. For several hours Thursday, jurors in the death-penalty trial of Dzhokhar Tsarnaev heard from an FBI agent who constructed models of the pressure cooker bombs that exploded at the Boston Marathon finish line nearly two years ago, and some seemed to lose interest. At least one juror refused to inspect the replica bomb when it was passed her way.

But with less than an hour left in the court day, the mood shifted when medical examiner Jennifer Hammers took the stand to describe how Krystle Marie Campbell was killed in the first explosion.

Her testimony included graphic, gut-wrenching photos, and many jurors cried when they viewed them.

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“The cause of death was . . . from blast injuries to her torso and her lower extremities,” said Hammers, who conducted the autopsy on Campbell’s body.

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Hammers, who now works in New York, explained that by “blast” she meant a “wave of energy that can cause injury to the body,” and described deep cuts to Campbell’s legs and torso, some as long as 10 inches. Campbell had third-degree burns on her back, and her hair was singed. BBs tore through muscle, and were found embedded in her body. There was black powder residue on her left hand.

Jurors appeared pained as they viewed the photos of Campbell’s body/injuries. One juror, who works in the field of energy resources, rested his forehead in his hand for a moment, before sitting up straight. He took a deep breath.

After 14 days of testimony, prosecutors have one last area of evidence to introduce: The cause of death of the three victims of the Marathon explosions.

Tsarnaev faces charges that include using a weapon of mass destruction, bombing a public place, and malicious destruction of property resulting in death, so prosecutors must prove that the bombs caused the deaths of Campbell, 29, of Arlington; Lingzi Lu, a 23-year-old graduate student; and Martin Richard, an 8-year-old boy from Dorchester.

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Prosecutors plan to present testimony and evidence about Lu and Richard’s death on Monday, when the trial resumes, and the jurors’ reaction to the photos of Campbell — which were not shown to the public — offer a glimpse of the affecting evidence the government must present before resting its case.

Tsarnaev, now 21, faces multiple charges that carry the possibility of death penalty, and though he has pleaded not guilty, he has admitted through one of his lawyers that he and his older brother set off the explosions, which also injured more than 260 people.

Before the medical examiner took the stand, jurors heard testimony Thursday from an FBI supervisory agent who created models of the bombs, based on “all the evidence” investigators found at the scene — thousands of “bits and pieces” of materials. Edward S. Knapp said authorities determined that the two bombs placed at the finish line were built with pressure cookers each holding more than eight pounds of explosive powder.

The powder was ignited with a wire for Christmas tree lights, which was connected to a receiver from a remote control car. The Tsarnaev brothers used remote transmitters from the same remote control car sets to send a signal to the receiver that lit the Christmas light wire, igniting the explosives.

Knapp said the bomb the brothers detonated during a confrontation with police in Watertown was built in a similar fashion, though the brothers lit a hobby wire fuse — likely with a lighter — to ignite the powder.

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Knapp displayed models of the bombs for jurors, explained that the bombs had contained shrapnel.

Prosecutors said the brothers learned how to build the bombs by reading Inspire magazine, an online publication sponsored by Al Qaeda in the Arabian Peninsula, which published an article titled, “How to make a bomb in the kitchen of your mom.”

“It’s not a difficult system to build,” Knapp testified. “It’s not that too sophisticated.”

Prosecutors allege that the Tsarnaev brothers used explosive powder from fireworks to build the bombs, though FBI chemist David McCollam testified that the brothers would have had to empty a dozen mortar-style fireworks just to produce a pound of explosive powder.

He also acknowledged that materials that were found in Tamerlan Tsarnaev’s room and in his car, such as rubber gloves, tested positive for the same explosive powder found at the bombing scene.

Milton J. Valencia can be reached at mvalencia@globe.com. Follow him on Twitter @miltonvalencia.