The Washington PostDemocracy Dies in Darkness

At the Boston Marathon bombing trial, harrowing testimony and detail about the explosive devices

Members of the media gather outside federal court on March 5, 2015, in Boston during the death penalty trial of Dzhokhar Tsarnaev. (Michael Dwyer/AP)

The jury in the Boston Marathon bombing trial is going home for the weekend after a day of some of the most frightening things being described in soft monotones. The last witness of the day — and probably one of the prosecution’s last witnesses in this phase of the trial — was medical examiner Jennifer Hammers, who performed an autopsy on one of the three bombing victims, 29-year-old Krystle Campbell.

Hammers gesticulated as she described Campbell’s injuries: the singeing of her hair, the bruises and scrapes to her head and face, the wounds to the back of her right upper arm, the pellets in her left hand. In fact, as the medical examiner methodically worked her way down, it seemed that there was no part of Campbell’s body that had not been affected.

The jury and the witness saw photographs of Campbell’s body, but, unlike other evidence in this case, the pictures were not shown to the public, apparently because they were deemed potentially too disturbing. Nor did the public see objects that Hammers identified as having removed from Campbell’s body; these included pellets, found both in the victim’s skin and deep inside her large wounds, pieces of silicone, and a shard of metal. Hammers said Campbell died of blood loss and probably lived for up to a minute after the blast.

The medical examiner’s testimony lasted only half an hour at the end of Thursday’s hearing. It was preceded by hours of testimony by FBI agent Edward Knapp, who described, in minute detail, the composition of the explosive devices he had reconstructed from more than 13,000 pieces of evidence collected at the sites of the blasts and the firefight with law enforcement in Watertown three days later.

The bombs were made using pressure cookers, gunpowder, Christmas lights, radio-controlled cars, pipes and food containers.

“How sophisticated do you have to be?” asked prosecutor William Weinreb.

“It’s not too complicated,” the agent responded. “Once you have the components of the hobby car… it’s not too difficult a system to build.” Detailed instructions are available on the Internet.

Knapp ran through slides from a set of instructions that appeared in Inspire magazine, the online publication produced by al-Qaeda’s affiliate in Yemen, which the prosecution has alleged the Tsarnaev brothers used to build the bombs. The instructions, titled “How to Build a Bomb in the Kitchen of Your Mom,” look and read like a cooking recipe and include phrases such as “mix well.”

“With that said, here are some important steps to take for an effective explosive device,” one of the last slides says.

“*Place the device in a crowded area.

“*Camouflage the device with something that will not hamper the shrapnel, such as cardboard.

“The pressurized cooker is the most effective method.”

After lunch, prosecutor Aloke Chakravarty got Knapp off the witness stand and into the courtroom, to demonstrate the mock-up explosive devices he had assembled based on the evidence. These, the agent confirmed, had been emptied of gunpowder before being brought into court.

The first mock-up looked like a pressure cooker with the innards of a radio-controlled car placed inside. The agent also had a brand-new black backpack like the ones the brothers had used to carry the bombs. At the prosecutor’s request, he placed the pressure cooker inside the backpack. Knapp then weighed the backpack in his right hand.

“It is pretty heavy,” he said.

Chakravarty then asked Knapp to take the mock-up bomb out of the backpack “so the jury can take a close-up look at it.” It still looked like a pressure cooker.

So did the second mock-up, and the third. The next two looked like the sort of pipe connectors used in household plumbing in places where pipes need to change direction. The sixth mock-up was made with a food container. At the end of the agent’s hours-long testimony, the jury was looking at three pressure cookers, two pipe connectors and one Rubbermaid container. It looked very much like “the kitchen of your mom.”