Who actually built the Boston Marathon bombs?

The question of whether Dzhokhar or Tamerlan Tsarnaev built the bombs that exploded at the finish line of the Boston Marathon was a major point of the trial Thursday.

US Attorney's Office/Reuters
This image shows an unexploded metal bomb filled with explosive powder and lined with metal pellets that was entered as evidence in the trial of Boston Marathon bombing suspect Dzhokhar Tsarnaev.

The federal court room hosting the Boston Marathon bombing trial has remained mostly silent for 14 days of testimony. Today that silence fell a little deeper as the jury spent the morning learning about the explosive devices at the heart of the case, then the afternoon examining mockups of the devices and hearing, in often gruesome detail, the physical damage the bombs caused.

The trial now stands a few witnesses away from the end of its first stage. The prosecution plans to call only two more witnesses in the first phase of the trial and could rest its case against the accused bomber, Dzhokhar Tsarnaev, as early as Monday, the next scheduled day of testimony.

Thursday’s testimony shifted sharply from cold and technical in the morning to raw and emotional in the afternoon.

The day began with Federal Bureau of Investigations forensic examiner Kevin McCollam, whose testimony focused on whether he could determine where the bombs were made. Mr. Tsarnaev is accused of carrying out the bombings with his older brother, Tamerlan, who died when he and Dzhokhar were attempting to escape police several days after the bombing. Determining which brother physically built the bombs could be important in determining whether Dzhokhar receives the death sentence.

“I do not, based on our analysis, think we can tell where these bombs were built,” Mr. McCollam told Assistant United States Attorney Aloke Chokravarty.

Most of the day was spent with Edward Knapp, a trained bomb technician and supervisory special agent with the FBI. Mr. Knapp analyzed more than 1,300 pieces of evidence as part of the investigations and built mockup replicas of the bombs used in the attacks.

Knapp stood directly in front of the jury – not in the witness box – and for an hour talked the jury through the assortment of mock pressure cooker and pipe bombs on a table in front of him.

Though the jury passed the mock bombs among one another, examining the circuitry and the nails and shrapnel taped to the inside of the containers, the testimony was mostly dry and technical. Knapp explained how transmitters for remote control cars were used to trigger shattered Christmas tree lights inside the devices, setting off the explosive material inside. When Knapp squeezed down on the mock transmitter, the mock pressure cooker bomb emitted a soft beep.

Prosecutors asked Knapp to read through instructions from Inspire magazine – a radical Islamic magazine found on various computers used by the Tsarnaev brothers – describing step-by-step how to make such devices.

The instructions, written in English, included suggestions to “place the device in a crowded area” and to “wear gloves” to avoid leaving fingerprints.

“Trust in Allah and pray for success of your operation,” the instructions added. “This is the most important rule.”

The most dramatic moments came during the testimony of the final witness of the day, Jennifer Hammers, a forensic pathologist with the Massachusetts medical examiners office during the bombings. 

Ms. Hammers discussed her autopsy of Krystle Campbell, one of the three victims killed by the bombs. At least one juror was apparently brought to tears during the testimony.

After the prosecution rests its case – presumably Monday – defense lawyers will then have the opportunity to call their own witnesses, but it is unclear if they will do so.

Tsarnaev faces 30 federal charges, 17 of which carry the death penalty. If the jury finds him guilty, the trial will move to a sentencing phase, where the same jury will decide to sentence him to death or life in prison.

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