By Meggie Quackenbush
BU News Service

For Shane O’Hara, manager of the Marathon Sports store on Boylston Street in Boston, the scene after the first bomb went off at the Boston Marathon finish line on April 15, 2013, is something he’s never going to forget.

“It looked like a scene out of Saving Private Ryan or Platoon,” O’Hara said, testifying in federal court on Wednesday in the trial of Dzhokhar Tsarnaev, the man alleged to have carried out the attacks. That first bomb went off directly outside Marathon Sports, which sits next to the finish line, wounding bystanders and killing 29-year-old Krystal Campbell.

“I remember trying to decide who needed help the most, who can I help, and why do I have to decide this?” O’Hara said, close to tears.

“I never would have thought I’d have to make that decision,” he said.

It was an emotional day of witness testimonies at Boston’s federal courthouse, where opening statements began in the trial. Tsarnaev stands accused of setting off bombs that killed three people and injured more than 250. If convicted, he faces the death penalty.

The day began with graphic details from the lead attorney for the prosecution William Weinreb, who described the bloody scene along Boylston Street in the aftermath of the bombings. He said that Lingzi Lu, a 23-year-old Boston University graduate student, just happened to be walking by Forum restaurant when the second bomb went off that day. She was “bruised, battered, and cut” by the shrapnel from the bomb, Weinreb said, and “laid screaming in pain” on the sidewalk while those around her tried unsuccessfully to save her life.

Weinreb described surveillance footage showing Tsarnaev slouching towards crowds of families along the finish line, backpack slung on one shoulder. The footage then shows him dropping that bag behind a row of children, said Weinreb. One of those children was Martin Richard, the 8-year-old whose “entire body” was damaged when the second bomb was detonated, he said.

“Martin bled to death on the sidewalk as his mother looked helplessly on,” he said.

And while paramedics, race volunteers, bystanders and police officers battled to save lives on Boylston Street in the hours after the attack, Weinreb said that Tsarnaev “pretended nothing was wrong.” The then 19-year-old UMass Dartmouth student went back to school, partied with friends and played video games, said Weinreb.

Tsarnaev did this because he “believed what he did was right,” said the prosecutor. “The defendant wanted to punish America for its actions overseas,” he said.

But according to the defense, the case against Tsarnaev doesn’t come down to whether or not he committed the crimes, which include using a weapon of mass destruction, just one of 30 federal charges against him.

“It was him,” said Judy Clarke, an attorney for Tsarnaev’s defense who is famous for keeping controversial figures like Jared Lee Loughner and Ted Kaczynski off of death row.

“The government and the defense agree about many things,” she said, but for the defense, “it is the why” where the two sides will disagree.

Clarke said the defense will argue that Tsarnaev was influenced by his obsessive, radicalized older brother Tamerlan, who lead the two on a “path of devastation” the day of the bombings and on a manhunt through Watertown three days later.

In a room filled with the victims and families affected by the bombings, Clarke described how it was Tamerlan, not Dzhokhar, who had disrupted services at his mosque when others wouldn’t agree with his radicalized views, who traveled abroad and returned with extreme Muslim beliefs, who purchased the pressure cookers, BBs, and electrical components to build the bombs.

According to the defense, Dzhokhar was just like any other teenager—more concerned with video games and girls than with the strict beliefs of his brother. That changed, Clarke said, as he began to struggle in school and became more vulnerable to Tamerlan’s influence.

“He came to his role by a very different path, created by his brother,” Clarke said.

While his brother had “immersed himself in death and destruction and carnage in the middle east,” Dzhokhar was much more concerned with the things that interest teenage boys—”Facebook, cars, and girls,” she said.

The defense opted not to cross-examine any of the witnesses who offered detailed, dramatic accounts on the opening day of testimony.

In their admission of his guilt, the defense now has the difficult task of persuading the jury that Tsarnaev was under the influence of an overbearing, obsessive brother at the time of the bombing in order to avoid the death penalty.

“You wouldn’t be sitting here today unless you convincingly told us throughout this phase you’d remain open,” Clarke reminded the jury.

“It’s going to be a lot to keep your hearts and minds open, but that’s what we ask,” she said.

Related: Live blog and updates from the trial