12:50 P.M.

Jurors in the trial of Boston Marathon bombing suspect Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv) have been shown a video of the first explosion.

The video shows a huge blast of smoke, then police officers running toward the blast. Runners looked over their shoulders and spectators turned their heads toward the sound of the bombing.

The jury watched the video intently, many looking somber.

Twenty-nine-year-old Krystal Campbell was killed by the first bomb. She was among the three killed and 260 injured after twin bombs exploded near the marathon finish line.

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12 P.M.

The first witness to testify is the executive director of the organization that puts on the marathon.

Thomas Grilk is the head of the Boston Athletic Association.

Grilk on Wednesday described the history of the marathon, beginning with the first race in 1897, when just 15 people ran. In recent years, tens of thousands of people have run the marathon.

Grilk said as many as a half million spectators turn out for the marathon each year, with people lining the streets in cities and towns along the 26.2-mile course and huge crowds gathering near the finish line in Boston.

Tsarnaev is accused of planting bombs near the marathon finish line in 2013, killing three people and injuring more than 260.

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11 A.M.

A lawyer for Tsarnaev has begun her opening statement by admitting her client participated in the attacks.

Judy Clarke says: "It was him."

She says the only thing the defense disagrees with prosecutors about is "why."

Clarke calls the bombings a "series of senseless, horribly misguided acts carried out by two brothers."

But she portrayed brother Tamerlan Tsarnaev as the ringleader, saying he planned it and "enlisted his brother into these series of horrific acts." Tamerlan died in a shootout with police.

Clarke says: "The evidence will not establish and we will not argue that Tamerlan put a gun to Dzhokhar's head or that he forced him to join in the plan, but you will hear evidence about the kind of influence that this older brother had."

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10:20 A.M.

A prosecutor says Tsarnaev's motive for the Boston Marathon bombing was to reach paradise by claiming victory in a holy war against Americans.

Assistant U.S. Attorney William Weinreb said Wednesday in opening statements that Tsarnaev "acted like he didn't have a care in the world" after the bombings. He says Tsarnaev hung out with friends while victims learned their limbs would have to be cut off.

The prosecutor described how 8-year-old Martin Richard stood on a metal barrier with other children so he could see the runners. Weinreb says Martin's mother found him lying on the ground bleeding to death after the bombs exploded.

The victims watched somberly as Weinreb described the carnage. Several victims hung their heads and appeared to fight back tears.

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10 A.M.

Tsarnaev's trial has begun with opening statements from the prosecution.

Weinreb said Wednesday that Tsarnaev carried a bomb in a backpack.

The prosecutor says: "It was the type of bomb favored by terrorists because it's designed to tear people apart and create a bloody spectacle."

Tsarnaev had no reaction and continued to look straight ahead — not at the jury, not at the prosecutor.

Weinreb continued: "The air was filled with the smell of burning sulfur and people's screams."

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9:45 A.M.

Jurors deciding Tsarnaev's fate have been sworn in and are listening to preliminary instructions from the judge.

Before the jury was brought in, Judge George O'Toole Jr. granted a prosecution motion to limit the amount of evidence the defense can present about the relationship between Dzhokhar Tsarnaev and his brother, Tamerlan, during the guilt phase of the trial. Tamerlan died during a gun battle with police days after the bombing.

O'Toole says some evidence of the brothers' interactions will be "inevitable," but most should be reserved for the second phase of the trial, when jurors will decide whether Tsarnaev receives the death penalty or spends the rest of his life in prison.

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9:20 A.M.

Tsarnaev is sitting alone at the defense table as his lawyers and prosecutors huddle at a sidebar with the judge. After a few moments, a member of his defense team goes to sit next to him.

Tsarnaev's hair is still untamed and curly, and he has a scraggly goatee. He's wearing a gray suit jacket and slacks.

The judge has formally denied the fourth change-of-venue motion filed Monday by the defense.

Tsarnaev's lawyers had acknowledged their three earlier requests were denied but said they wanted to complete the record of their opposition. They have argued their client couldn't get a fair trial in Massachusetts because of the publicity of the case and the proximity to the attacks.

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7:30 A.M.

Victims of the Boston Marathon bombing have arrived at court for opening statements in Tsarnaev's trial.

Two busloads of people hurt in the bombings arrived at the federal courthouse at 7:30 a.m. Monday. They entered through a side entrance, away from reporters and photographers gathered at the main entrance.

Marc Fucarile (FYOO'-kuh-rihl) went in the front entrance but did not comment to reporters. He lost a leg in the attack.

Lawyers for Tsarnaev have made it clear they will try to show that he was influenced by his older brother, Tamerlan, who died following a shootout with police days after the bombings.

Prosecutors say Dzhokhar was an equal and willing participant in the plot that killed three and hurt more than 260.