Gruesome Details Open Boston Bombing Trial

A courtroom sketch shows Boston Marathon bombing suspect Dzhokhar Tsarnaev (R) during the jury selection process in his trial at the federal courthouse in Boston, Massachusetts January 15, 2015. Jane Flavell Collins/Reuters

BOSTON (Reuters) - Clouds of smoke, improvised tourniquets and screams of pain were evoked in the first day of the Boston Marathon bombing trial on Wednesday as people who survived the deadly attack recalled one of the darkest days in the New England city's memory.

One survivor testified about being hurled through the air by the blasts that killed three people and injured 264 on April 15, 2013. A store manager recalled tying an improvised tourniquet on a wounded woman. And the mother of a 5-year-old boy recalled seeing bones sticking out of her arm.

Sydney Corcoran, a young woman who had gone to the race with her parents to watch her aunt run, recalled being enveloped in smoke and finding herself lying on the ground surrounded by people tending her injuries.

"I remember thinking that, this was it. I'm going to die. I'm not going to make it," said the college sophomore, who lost a large amount of blood from shrapnel wounds to her right leg.

"I remember having moments of panic when I would look around and I would see carnage and just blood on the street," she testified. I had no idea what had happened, I just knew it was bad."

She did not lose the leg. But her mother, Celeste, lost both legs in the attack.

Attorneys for Dzhokhar Tsarnaev, 21, opened the U.S District Court trial with the stark admission that their client was responsible for the bombing and the murder later in the week of a police officer.

But they said his actions were driven by his 26-year-old brother, Tamerlan, in a bid to reduce Dzhokhar's culpability in the jury's eyes and avert a death sentence.

The jury was shown several graphic videos, including the widely broadcast images of the smoke plumes that erupted from the twin bombs and of the chaotic and bloody scenes that followed.

In what appeared to be a nod to the intense emotions summoned by the survivors' testimony, Tsarnaev's attorneys declined to cross-examine the first victims who testified.

The manager of Marathon Sports, Shane O'Hara, 44, recalled stepping to his front door to watch runners who had bought sneakers from him cross the finish line and later seeing his shop window wrapped in smoke.

Federal prosecutors showed O'Hara and the jury surveillance camera video taken inside his sporting-goods store. In the video, he is shown tying an improvised tourniquet on a woman hurt by the blasts and later carrying out stacks of clothing to be used as bandages.

"The thing that haunts me is the making decisions about who needed help first, who needed more, who was more injured than the other," said O'Hara, choking up. "That was never my role, to make that decision, but you felt like you had to do that."

Colton Kilgore of Asheville, North Carolina, who had traveled to Boston to watch his mother-in-law run, recalled being thrown through the air by the blasts. As he stayed with some of his more seriously injured family members, who lay in puddles of blood, he took video of the aftermath.

In one of Kilgore's videos shown to the jury, an unidentified first responder screamed, "She's mobile, she's mobile, get her out of here!", recalling the uncertain moments after the two blasts when it was unclear whether more bombs would follow.

"I'm worried about secondaries, the first responder said in the video.

Thomas Grilk, executive director of race organizer the Boston Athletic Association, testified. He recalled the magnitude of the blasts and the scores of medical volunteers, who normally tend to athletes suffering from dehydration and blisters, and who rushed many of the wounded to the race's medical tent.

"Those people did a magnificent job," Grilk said. "Everyone who entered that tent alive is alive today."