BOSTON — After 10 weeks of heart-wrenching and often gruesome testimony from more than 150 witnesses, including survivors with missing limbs and an anguished father who spoke of watching his young son die on the sidewalk in front of him, a jury sentenced Dzhokhar Tsarnaev to the death penalty for his role in the deadly 2013 Boston Marathon bombing.

Tsarnaev offered no visible reaction, though he glanced toward the jurors as they were individually polled on whether they supported the penalty of death. Some of the jurors, a man and at least two women, were crying.

The decision came a little over two years after a pair of pressure-cooker bombs ripped through a crowd of unsuspecting spectators near the marathon’s finish line in April 2013, killing three and injuring nearly 300. Among the injured: 17 amputees, many of whom took the stand against Tsarnaev with bomb shrapnel still embedded in their bodies.

The same jurors — seven women and five men — convicted Tsarnaev on April 8 on all 30 counts related to the bombings, including the shooting death of a Massachusetts Institute of Technology police officer days after the attacks. They then heard roughly three weeks of testimony in the penalty phase of the case, in which they were asked to determine whether to sentence Tsarnaev to life in prison without parole or the death penalty for his role in the bombings

Though Tsarnaev pled not guilty, Judy Clarke, his attorney, admitted her client’s role on day one of the first phase of the trial in March and repeatedly reiterated it, right up until the closing statements in the penalty phase. “I’m not asking you to excuse him,” Clark told jurors. “There are no excuses. I’m not asking you for sympathy.”

But Clarke did plead for “mercy” for her client, asking jurors to spare his life in spite of the “senseless and catastrophic acts” he committed. She cast Tsarnaev, now 21, as a troubled teenager from a dysfunctional family who came under the sway of his radicalized older brother, Tamerlan. The defense argued Tamerlan Tsarnaev, who died from wounds sustained during a shootout with police days after the bombings, plotted the attack and built the bombs — and Dzhokhar Tsarnaev, who had been abandoned by his mentally ill parents and was flunking out of college, merely followed.

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“If not for Tamerlan, this wouldn’t have happened,” Clarke said. “Dzhokhar would never have done this but for Tamerlan. The tragedy would never have occurred but for Tamerlan. None of it.”

Though Tsarnaev did not take the stand on his behalf and often appeared dispassionate in court, even during the most emotional and horrific testimony, the defense called as its final witness Sister Helen Prejean, a Catholic nun and death penalty opponent who inspired the film “Dead Man Walking.” She told the jury of several meetings she had with the convicted bomber starting right before the trial and how, in one meeting, he had expressed sympathy for the bombing victims. “He said it emphatically. He said, ‘No one deserves to suffer like they did,’” Prejean testified.

But government prosecutors ridiculed Prejean’s testimony, implying she would say anything to prevent the death penalty. Tsarnaev’s statement to her, they argued, merely echoed a note the bomber left in a Watertown, Mass., boat before he was captured, in which he wrote that he didn’t “like killing innocent people,” but that it was “allowed.”

“The fact that now, while he’s on trial for his life, the defendant is willing to go so far as to say that no one should have to suffer like that doesn’t tell you much about his core beliefs,” prosecutor William Weinreb told the jury. “When you stack that up against his actions, does it really make a difference to your decision?”

Throughout the trial, prosecutors painted Tsarnaev as a cold-blooded killer who deceived even his closest friends about his jihadist leanings and remains unrepentant about what he did. They argued he was an “equal partner” who walked in lockstep with his brother to carry out an attack aimed at inflicting terror and mayhem at one of Boston’s most celebrated public events to avenge the deaths of Muslims in wars overseas.

The government repeatedly showed the jury surveillance video of Tsarnaev dropping a backpack that contained one of the bombs behind the family of 8-year-old Martin Richard, the youngest victim of the bombings, and of him casually buying milk 20 minutes after the attack. They pointed to video of Tsarnaev flashing the middle finger to a security camera in a court holding cell before his July 2013 arraignment as proof that he remains defiant.

“No remorse, no apology,” Steven Mellin, another prosecutor, argued. He insisted there was no other “just” punishment for what Tsarnaev did than the death penalty.

On Friday, some jurors seemed sympathetic to the defense argument. Three of the 12 jurors said they agreed with the defense's mitigating argument that Tamerlan Tsarnaev had led his brother down a path of radicalization. Two said they believed Dzhokhar Tsarnaev had shown remorse for his crime. But it wasn't enough. All 12 agreed on the death penalty.

The jury’s decision to condemn Tsarnaev to death was a huge victory for the federal government, who pursued the sentence in spite of misgivings among some victims and family members of those killed. Among the most notable opponents of the death penalty: Richard’s parents, who, in a statement published on the front page of the Boston Globe, pleaded for prosecutors to accept a plea deal of life in prison for Tsarnaev to “end the anguish” of the trial and likely years of appeals. There was also strong opposition from residents of Boston, where many people oppose the death penalty on moral or religious grounds. Even after some of the most heinous testimony in the trial, a WBUR poll of Boston residents found that 62 percent of them favored a life sentence for Tsarnaev.

Tsarnaev will be formally sentenced at a hearing in coming weeks, where victims will be allowed to give impact statements and address the defendant. And Tsarnaev, too, will be given the opportunity to speak — though it’s unclear if he will.

Even at the conclusion of the nearly three-month-long trial, there were still many mysteries around the bombing plot. Prosecutors never said where the two pressure-cooker bombs were built — though it was strongly implied it was at the Tsarnaev family apartment in Cambridge.

There was also the mystery of Katherine Russell, Tamerlan Tsarnaev’s wife, and what, if anything, she knew of the plot. According to defense evidence, Russell was copied on many emails that Tamerlan Tsarnaev sent to his brother containing jihadist videos and writings. And in early 2012, just after Tamerlan Tsarnaev traveled to Russia to pursue jihad, someone using Russell’s computer searched phrases including “rewards for wife of mujahedeen” and “If your husband becomes a shahid, what are the rewards for you?”

Through her attorney, Russell has denied knowledge of the plot. Though her mother, best friend and former roommate testified, Russell was never called as a witness in the trial, and she has never been formally cleared by federal investigators.

Perhaps the biggest enigma of all remains Tsarnaev. Though his defense team went into great detail about his family’s troubles — including a father who was so mentally ill he saw imaginary lizards crawling on his body — the jury learned more about the motivations of Tsarnaev’s older brother, Tamerlan, than his.

A litany of former teachers and friends testified, often tearfully, about the kind and gentle “Jahar” they had known — a smart kid who seemed to thrive in spite of his dysfunctional family. Sitting in court just feet away from him, they stared at Tsarnaev and expressed shock that the boy they knew committed one of the most horrific crimes Boston has seen. They testified about Tsarnaev’s hopes and dreams, how he’d talked about being an engineer or becoming an attorney.

While the jury heard testimony about the relationship between Tsarnaev and his domineering older brother, Tamerlan, the defense did not definitively answer the questions of why and how a 19-year-old college kid who spent most of his time smoking pot and playing video games with his friends came to be a terrorist.

“If you’re looking to me for a simple and clean answer as to why this young man, who had never been arrested, who had never sassed a teacher, who spent his free time in school working with disabled kids … if you expect me to have an answer, a simple, clean answer as to how this could happen, I don’t have it,” Clarke told jurors in her closing statement. “I don't have it."

If he had stayed on track, Dzhokhar Tsarnaev likely would have graduated with the rest of the class of 2015 at the University of Massachusetts Dartmouth, which held its commencement ceremonies Friday. But instead, he was in a federal courtroom in Boston, learning that his life was over.