BOSTON — A jury sentenced Dzhokhar Tsarnaev to death Friday for the Boston Marathon bombing, sweeping aside pleas that he was just a “kid” who fell under the influence of his fanatical older brother.

Tsarnaev, 21, stood with his hands folded, his head slightly bowed, upon learning his fate, decided after 14 hours of deliberations over three days. It was the most closely watched terrorism trial in the U.S. since the Oklahoma City bombing case two decades ago.

The decision sets the stage for what could be the nation’s first execution of a terrorist in the post-9/11 era, though the case is likely to go through years of appeals. The execution would be carried out by lethal injection.

The 12-member jury had to be unanimous for Tsarnaev to get the death penalty. Otherwise, he would have automatically received a sentence of life in prison without parole.

Tsarnaev’s father, Anzor Tsarnaev, reached by phone by the Associated Press in the Russian region of Dagestan, let out a deep moan upon hearing the news and hung up.

Three people were killed and more than 260 wounded when two pressure-cooker bombs packed with shrapnel exploded near the finish line on April 15, 2013.

The former college student was convicted last month of all 30 federal charges against him, including use of a weapon of mass destruction and the killing of an MIT police officer during the Tsarnaev brothers’ getaway attempt. Seventeen of those charges carried the possibility of the death penalty.

Tsarnaev’s chief lawyer, death penalty specialist Judy Clarke, admitted at the very start of the trial that he participated in the bombings, bluntly telling the jury: “It was him.”

But the defense argued that Dzhokhar was an impressionable 19-year-old who was led astray by his volatile and domineering 26-year-old brother, Tamerlan, who wanted to punish the U.S. for its wars in Muslim countries.

Prosecutors portrayed Dzhokhar Tsarnaev as an equal partner in the attack, saying he was so heartless he planted a bomb on the pavement behind a group of children, killing an 8-year-old boy.

To drive home their point, prosecutors cited the message he scrawled in the dry-docked boat where he was captured: “Stop killing our innocent people and we will stop.” And they opened their case in the penalty phase with a startling photo of him giving the finger to a security camera in his jail cell months after his arrest.

“This is Dzhokhar Tsarnaev —unconcerned, unrepentant and unchanged,” prosecutor Nadine Pellegrin said.

The jurors also heard grisly and heartbreaking testimony from numerous bombing survivors who described seeing their legs blown off or watching someone next to them die.

Killed in the bombing were Lingzi Lu, a 23-year-old Boston University graduate student from China; Krystle Campbell, a 29-year-old restaurant manager from Medford; and 8-year-old Martin Richard, who had gone to watch the marathon with his family. Massachusetts Institute of Technology police Officer Sean Collier was shot to death in his cruiser days later. Seventeen people lost legs in the bombings.

Tamerlan Tsarnaev died days after the bombing when he was shot by police and run over by Dzhokhar during a chaotic getaway attempt.

In deciding on the death penalty, the jury had to fill out a detailed, 24-page worksheet in which they tallied up the so-called aggravating factors and mitigating factors.

The possible aggravating factors cited by the prosecution included cruelty of the crime, the killing of a child, the amount of carnage inflicted, and any lack of remorse. The possible mitigating factors included his age, the possible influence of his brother and his turbulent, dysfunctional family.

The jury agreed with the prosecution on 11 of the 12 aggravating factors they cited, including a lack of remorse. In weighing possible mitigating factors, only three of the 12 jurors found he acted under the influence of his brother.

Tsarnaev did not take the stand at his trial, and he slouched in his seat through most of the case, a seemingly bored look on his face. In his only flash of emotion during the months-long case, he cried when his Russian aunt took the stand.

The only evidence of any remorse on his part in the two years since the attack came from the defense’s final witness, Sister Helen Prejean, a Roman Catholic nun and staunch death penalty opponent made famous by the movie “Dead Man Walking.”

She quoted Tsarnaev as saying of the bombing victims: “No one deserves to suffer like they did.”

Tsarnaev’s lawyers also called teachers, friends and Russian relatives who described him as a sweet and kind boy who cried during “The Lion King.” The defense called him a “good kid.”

The defense argued that sparing his life and instead sending him to the federal Supermax prison in Florence, Colorado, would be a harsh punishment and would best allow the bombing victims to move on with their lives without having to read about years of death penalty appeals.

U.S. District Judge George O’Toole Jr. will formally impose the sentence at a later date during a hearing in which bombing victims will be allowed to speak. Tsarnaev will also be given the opportunity to address the court.

The Tsarnaevs —ethnic Chechens — lived in the former Soviet republic of Kyrgyzstan and the volatile Dagestan region of Russia, near Chechnya, before moving to the U.S. about a decade before the bombings. They settled in Cambridge, just outside Boston.

DENISE LAVOIE, AP Legal Affairs Writer