U.S. District Court Judge George A. O'Toole gives jurors instructions to follow in their deliberations in the trial of Dzhokhar Tsarnaev. (Richard Johnson/The Washington Post)

Federal prosecutors on Monday described the young man accused of carrying out the deadly bombing of the Boston Marathon as a coldblooded killer, as they sought in closing arguments to portray him as a determined terrorist seeking to commit violent jihad.

Assistant U.S. Attorney Aloke S. Chakravarty told the jury that Dzhokhar Tsarnaev brought terrorism to the streets of Boston to “awaken the mujahideen” and “punish America” on a day he knew the world would be watching.

Tsarnaev, 21, sat tucked between his lawyers as the veteran prosecutor laid out evidence against him, including footage from a surveillance camera that showed him planting one of the two bombs used in the attack. The footage depicted the aftermath of the heavy blast, with the wounded crawling to safety and first responders rushing to their aid.

“The evidence I will show you will give you the confidence” to convict Tsarnaev, the prosecutor said before playing the graphic video.

Tsarnaev, along with his older brother, Tamerlan, 26, detonated the shrapnel-filled bombs near the marathon finish line on April 15, 2013. The bombings killed three people and injured more than 200 others, sparking havoc among spectators who had gone to the marathon to cheer on friends and family.

Assistant U.S. Attorney Aloke Chakravarty makes his closing arguments before a jury decides whether Dzhokhar Tsarnaev, 21, should be convicted in connection with the April 15, 2013, Boston bombings. (Richard Johnson/The Washington Post)

“He chose a day there would be civilians on the sidewalk,” Chakravarty said. “He and his brother wanted to target those civilians.”

The prosecution called more than 90 witnesses over the course of the trial at a federal courthouse at Boston Harbor, several miles from the scene of the attacks. Tsarnaev’s lawyers have conceded his involvement in the bombings. But they have also argued that he was under the influence of his brother, whom they have characterized as the mastermind of the bombings.

“We need to understand who was leading and who was following,” his lawyer, Judy Clarke, told jurors Monday.

Tsarnaev’s defense lawyers are hoping to spare their client the death penalty. Clarke emphasized that Tsarnaev’s older brother was the one who bought the items used in the attacks: pressure cookers, the objects used as shrapnel, parts to remotely detonate the bombs and backpacks to carry them in. She said Tamerlan, not Dzohkhar, selected the Boston Marathon as the target.

“We ask you to hold your minds open,” she said about the penalty phase of the trial, which court officials expect to begin the last week in April. “We are not asking you to go easy on Dzhokhar. Your verdict will speak the truth.”

Tsarnaev is also charged with killing a Massachusetts Institute of Technology police officer as he and his brother attempted to evade authorities. Chakravarty, the federal prosecutor, noted that Sean Collier was shot five times, three of the bullets piercing him in the head and one between the eyes. He acknowledged that the prosecution did not know definitively who shot the officer, but said it didn’t matter who actually pulled the trigger, adding that both brothers were “equally guilty.”

“This was a cold, calculated terrorist act,” he said in describing the attacks. “It was bloodthirsty.”

Dzhokhar Tsarnaev has pleaded not guilty, but don't expect his lawyers to claim he wasn't involved in the 2013 Boston Marathon bombing. Here's what to expect inside the courtroom. (Gillian Brockell/The Washington Post)

Chakravarty disputed the claim that Tsarnaev was simply under the influence of his brother, portraying him as an active and willing participant who was eager to launch the devastating attack. He described the brothers as partners, working in unison as they carried out the attack and then moved to flee.

“They were a team,” the prosecutor said. “That’s how they rolled.”

Chakravarty recounted how Tsarnaev acquired the handgun used in a carjacking and shootout with police.

“They each had their roles,” the prosecutor said. Tsarnaev had “done his job well.”

Tamerlan Tsarnaev was killed days after the bombings in a violent confrontation with police after authorities identified the brothers as possible suspects.

A badly wounded Dzhokhar Tsarnaev, then 19, managed to get away and hide in a boat next to a house in Watertown, Mass. Police officers later descended on the boat and arrested him.

Before his arrest, Tsarnaev scrawled a message with a pencil in the boat — which was riddled with bullet holes — denouncing the U.S. government for killing Muslims and mourning the death of his brother. He was jealous that his brother had died as a martyr and had reached “the highest levels of heaven,” Chakravarty said.

Among the witnesses to testify at the trial was the father of 8-year-old Martin Richard, who was killed after one of the bombs exploded just feet from him and his family. Prosecutors say Tsarnaev planted the bomb that ripped apart the boy and injured his mother, father and sister, who lost a leg in the attack.

Clarke, the defense lawyer, argued that her client intentionally placed the bomb behind a tree and did not target Martin.

Martin’s father testified that he knew his son would not survive the explosion after spotting him through the smoke. “I just knew from what I saw that there was no chance,” Bill Richard said.

Martin’s youth will most likely be cited by prosecutors as an aggravating factor among others in support of the death penalty.