As he planted a backpack containing a bomb just feet from a group of children, Dzhokhar Tsarnaev made a cold-hearted decision aimed at spreading terror, a federal prosecutor told jurors during closing arguments at Tsarnaev's death penalty trial.
There was nothing about this day that was a twist of fate," said assistant US attorney Aloke Chakravarty. "This was a cold, calculated terrorist act. This was intentional. It was bloodthirsty. It was to make a point. It was to tell America that 'We will not be terrorised by you any more. We will terrorise you.'"
Defence lawyer Judy Clarke countered by arguing, as she did at the trial's outset, that Tsarnaev took part in the attack but that he did so under the malevolent influence of his now-dead older brother, Tamerlan.
Ms Clarke repeatedly referred to Dzhokhar Tsarnaev as a "kid" and a "teenager".
"We don't deny that Dzhokhar fully participated in the events, but if not for Tamerlan, it would not have happened," Ms Clarke said.
The jury was expected to begin deliberating as early as this afternoon in the case against Tsarnaev, 21, almost two years after the twin bombings near the finish line of the Boston Marathon killed three people and wounded more than 260 in the nation's deadliest terror attack since the September 11 terrorist attacks.
If Tsarnaev is convicted - and that is considered a near certainty, given his lawyer's admission - the jury will begin hearing evidence on whether he should get life in prison or a death sentence.
Prosecutors used their closing argument to remind the jury of the horror of that day, showing photographs and video of the carnage and chaos after the twin pressure-cooker bombs exploded. In one video, jurors could hear the agonising screams of Krystle Campbell, a 29-year-old restaurant manager who bled to death on the pavement.
Taking aim at the argument that Dzhokhar Tsarnaev was led astray by his older brother, Mr Chakravarty repeatedly referred to the Tsarnaevs as "a team" and "partners" in the attack.
"That day, they felt they were soldiers. They were the mujahedeen, and they were bringing their battle to Boston," the prosecutor said.
As for the youngsters killed or maimed by the bomb that was in Dzhokhar's backpack, Mr Chakravarty said: "These children weren't innocent to him. They were American. Of all the places that he could have placed the bomb, he placed it right there."
Tamerlan Tsarnaev, 26, died four days after the bombings after he was shot by police and run over by Dzhokhar during a getaway attempt. Dzhokhar was captured hiding in a dry-docked boat.
At the end of his closing argument, Mr Chakravarty displayed photos of the three people killed in the bombings - including eight-year-old Martin Richard - and the Massachusetts Institute of Technology police officer who was shot to death during the getaway attempt.
"They are no longer with us," Mr Chakravarty said. "This is the result of the defendant's choice to be a terrorist, his choice to make a statement. These were choices that he was proud of."
Clarke took a conciliatory tone in her closing argument, admitting the attack brought "tragedy, suffering and grief in dimensions that none of us could imagine were possible".
And she said Tsarnaev "stands ready by your verdict to be held responsible for his actions".
But Ms Clarke also said Tamerlan played a much more prominent role in the attack, buying bomb components, including pressure cookers, BBs and remote control parts.
She said Tamerlan researched via computer how to build the bombs and planned the attack. And his fingerprints - but not Dzhokhar's - were found on pieces of the two bombs.