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Boston Marathon bombing trial begins for Dzhokhar Tsarnaev

Jurors watched video footage of the explosions and heard from a witness who struggled to help people caught in the grisly mayhem.

BOSTON — Nearly two years after twin blasts rocked the finish line of the Boston Marathon, alleged bomber Dzhokhar Tsarnaev appeared before a jury Wednesday as a federal prosecutor accused him of believing he was "a soldier in a holy war against Americans" while Tsarnaev's defense lawyer admitted that "it was him" in surveillance videos dropping a bomb-laden backpack on crowded Boylston Street.

In opening statements, Assistant U.S. Attorney William Weinreb painted a detailed picture of the moments before the April 15, 2013 attack, with thousands gathered along the race sidelines as a Boston Red Sox game let out. Weinreb also presented a portrait of Tsarnaev as a duplicitous character who kept his violent tendencies secret from his closest friends and mingled among spectators before unleashing carnage.

"He wasn't there to watch the marathon. He had a backpack over his shoulder and inside that backpack he had a bomb," Weinreb said. That bomb, made of a pressure cooker, was "the type of bomb preferred by terrorists."

Jurors later watched video footage of the explosions and heard from a witness who struggled to help people caught in the grisly mayhem.

The trial began with Tsarnaev, wearing a striped shirt and dark blue sports jacket, sitting motionless and staring forward as Weinreb walked jurors through the government's account. The jurors remained riveted on the prosecutor, who sought to portray the bombings as the result of a deliberate plan hatched by Tsarnaev and his older brother, Tamerlan, who died in a shootout with police.

Weinreb went on to describe how Dzhokhar Tsarnaev walked in front of the Forum restaurant and set the backpack "right behind a row of children." One of those children, 8-year-old Martin Richard, would die from the blast after Tsarnaev allegedly used a remote control to detonate the device.

The bombs, a block apart, exploded simultaneously, killing three people: Richard, 29-year-old Krystle Campbell, and 23-year-old Lingzi Lu. In the manhunt that followed, an MIT police officer, 26-year-old Sean Collier, was shot to death, allegedly by the brothers.

Four days after the attack, Tsarnaev, 19 at the time, was found hiding in a boat stored in a backyard, wounded by police gunfire.

Jurors spent part of Wednesday's testimony watching video footage of the bombings, the first outside a running shoe store, Marathon Sports, the second in front of the Forum, and the aftermath as runners scattered and police officers ran into the street. Tsarnaev maintained his expressionless gaze.

The visceral imagery in the trial's opening minutes was just a prologue to what is expected to be a painstaking retelling of an attack that will reawaken horrific memories and raise the possibility of a death penalty case in a state that hasn't executed anyone since 1947.

Tsarnaev's lead defense lawyer, Judy Clarke, did not dispute many of the prosecution's main points. "We do not attempt to sidestep Dzhokhar's responsibility for his actions in his trial," she told the jury in her opening statements.

The trial is expected to hinge less on Tsarnaev's guilt — to be determined in the first phase — as much as whether he gets the death penalty — to be determined in the second phase.

Clarke conceded that her client was caught on surveillance cameras dropping the backpack, and that the bomb and a second one detonated by Tamerlan Tsarnaev, "extinguished three lives." She described the bombings and the deadly manhunt that followed as "a series of senseless acts carried out by two brothers."

But Clarke also sought to portray Dzhokhar Tsarnaev as a follower of his older brother, who masterminded the plot. Clarke showed a photo of the brothers when they were much younger, and asked jurors to imagine how the younger Tsarnaev became so violent. She answered by saying Tamerlan was the one who bought the pressure cookers, BB gun pellets and fireworks used to assemble the bombs.

Dzhokhar Tsarnaev's path was‎ "a path borne of his brother, paid by his brother," Clarke said. Dzhokhar, she said, "bought into his brother's plan."

She concluded, "It is going to be a lot to ask of you but we ask you to keep your hearts and minds open."

Some of Tsarnaev's relatives and many of his alleged victims sat in the audience. To Tsarnaev's left sat 12 jurors and six alternates, chosen in a two-month selection process that included multiple requests by defense lawyers to move the trail out of Boston, where they said Tsarnaev couldn't get a fair trial in a city where nearly everyone was impacted by the bombing or its aftermath.

Weinreb, in his opening statements, led jurors through what he described as Tsarnaev's increasing attraction to Islamic militancy in the months before the attack: an interest in "terrorists' music and songs," researching bomb-making, collecting a "virtually complete" library of the al Qaeda-linked magazine Inspire, the purchase of a handgun.

The prosecutor also walked jurors through the government's version of the hunt for the Tsarnaev brothers in the days after the bombing: the killing of Collier with a shot between the eyes, the carjacking of an SUV, a police chase, a gunfight in which the brothers tossed two pressure-cooker bombs at officers. "It exploded with a thunderous boom and shrapnel rained down on the officers," Weinreb said.

After opening statements, the prosecution began its case by calling its first witness, Thomas Grilk, executive director of the Boston Athletic Association, which operates the Boston Marathon. The race, held on Patriot's Day on the third Monday in April, is a local holiday marking the battle of Lexington and Concord, with schools out and many workers with the day off.

Grilk said that more than 27,000 runners competed in the 2013 race, with another half-million spectators and 8,500 volunteers packed along the route. That year's race funneled about $142 million into the local economy, he said.

Jurors were shown photographs and diagrams of the finish line, and a video of the race.

The weather on April 15, 2013 was unseasonably warm — "a good day to run and a good day to watch," Grilk said.

Grilk described the finish line as a crowded, boisterous atmosphere, which as a former marathon runner he always found inspiring. "It was as dense here as it gets," he said. "People fill the sidewalks."

Grilk was followed by Shane O'Hara, a store manager at Marathon Sports, who said he was standing right outside the entrance when a bomb went off, shattering a nearby pane of glass and kicking up a cloud of dust. Jurors were shown an accompanying video that showed O'Hara trying a tourniquet around an injured woman's leg, with screams for help in the background. Then they were shown a photo of the bloody scene from above, with O'Hara in the thick of it.

"Things that haunt me is making decisions on who needed help first," O'Hara said, nearly overcome with emotion. "It was a scene like Saving Private Ryan or Platoon, something you thought you would never see in your life."

— with Andy Thibault

This story originally appeared on NBC News.