Metro

Marathon bombing trial to take technical turn

Expert witnesses will follow chilling images

They all remember the thunderous boom, the white smoke, the smell of sulfur and gunpowder.

Survivors of, and responders to, the Boston Marathon bombings who took the witness stand in the first days of the trial of Dzhokhar Tsarnaev have provided consistent, vivid accounts of the scene — what they smelled, heard, and saw — in the initial moments after the bombs exploded.

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In the next phase of testimony, those accounts will probably take on added importance, showing how investigators used witnesses’ scene-based evidence to help determine the technical nature of the bombs — their force, structure, and origin.

The technical testimony may not be as riveting as some of the personal accounts offered from the witness stand in the opening days, but it is essential to building an informed case against the bombers.

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Even though Tsarnaev has, through his lawyers, acknowledged his role in the bombings, law enforcement officials still have to show the jury how the bombs were built and how they were detonated. In the process, investigators will describe how they conducted their work.

“You’re looking at the color of the smoke, the amount of damage, the type of container that was used, the sound,” said Michael Bouchard, president of Security Dynamics Group, a security consultant company, who retired after 20 years with the US Bureau of Alcohol, Tobacco, Firearms and Explosives. A former head of the ATF’s national response team, he was not involved in the Marathon bombing investigation, but he described what the work would have included, and what was required, in the hours and days after the blasts.

He said a team of crime scene investigators from the FBI, the ATF, and the Boston Police Department, among other agencies, would have examined not only the witnesses’ accounts of the color of the smoke and the sound of the explosion, but the damage to the ground at the sites of detonation.

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Investigators would have also wanted to see how far the shrapnel was scattered and whether it reached any roofs.

The type of bombs would have told investigators of the nature of the attacks: Were they military-grade explosives or explosives made with items commonly on the commercial market?

“You have to go in there quickly, do a quick analysis, to get an idea that this is the container that was used, the explosive that was used, and you get your investigative leads from there,” he said. “But it’s extremely time-consuming, painstaking detail. There are multiple things going on at once.”

The dual explosions killed three people and injured more than 260. Tsarnaev, 21, faces the possibility of the death penalty, and his lawyers are attempting to use the trial to show he is less culpable than his older brother, who was killed days after the bombing during a confrontation with police in Watertown. Tsarnaev’s lawyers hope a federal jury will spare him from the death penalty.

Tsarnaev faces 17 charges that carry the possibility of the death penalty, such as using a weapon of mass destruction, resulting in death.

Already, officials have said in court documents that the brothers reconfigured pressure cookers and filled them with low-explosive powder and shrapnel, such as BBs. Tamerlan used powder from fireworks he bought in New Hampshire for the explosives.

Authorities said the brothers learned how to build the bombs from an Al Qaeda-sponsored online publication.

‘You just hear stifled screams of people you can’t see . . . all you see is smoke.’ --Sydney Corcoran, Lowell 19-year-old who attended the race with her mother

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Devices “constructed in this manner are designed to shred flesh, shatter bone, and cause extreme pain and suffering, as well as death,” according to the federal indictment in the case.

Bouchard said the authorities’ description of the bombs matches what the victims and responders testified last week to seeing and hearing.

Colton Kilgore from North Carolina, who was at the Marathon to watch his mother-in-law at the finish line when the first bomb went off, recalled the smell of gunpowder and the smoke, “just acrid and disgusting.”

“You just hear stifled screams of people you can’t see . . . all you see is smoke,” said Sydney Corcoran, 19, of Lowell, who was at the race with her mother.

Jeff Bauman, who lost both legs in the bombings, said, “I saw a flash, I heard three or four pops, and I was on the ground.” He recalled thinking, “That was a big firework. My ears were ringing, everything was muffled.”

Boston Police Officer Thomas Barrett said the first bomb sounded like a cannon. Then, at the site of the second explosion, he said he saw “an orange fireball,” followed by white smoke, then black smoke.

Testimony is expected to resume Monday.

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