The jury found that death was the appropriate punishment for six of 17 capital counts — all six related to Tsarnaev’s planting of a pressure-cooker bomb, which his lawyers never disputed. Tsarnaev sat stone-faced as the verdict was announced.

At the same time, the jury rejected the centerpiece of the defense argument, that he was under the influence of his older brother, Tamerlan, a self-radicalized jihadi. Nor did it believe that being locked away in the supermax prison in Colorado would sufficiently restrict Tsarnaev’s communications with the outside world.

Only two of the 12 jurors said on the verdict form that they believed he had expressed sorrow and remorse for his actions, a stinging rebuke to Sister Helen Prejean, a Roman Catholic nun who had testified for the defense that Tsarnaev was “genuinely sorry” for what he had done.

The bombings transformed the marathon, a cherished rite of spring, from a sunny holiday on Boylston Street to a smoky battlefield scene, with shrapnel flying, bodies dismembered and blood saturating the sidewalks; three people were killed, while 17 people lost at least one leg. More than 240 others sustained serious injuries, some of them life-altering.

The packed courtroom was silent throughout the proceedings — warned before the judge and jury entered that any outburst would amount to contempt of court.

When the jury returned, the forewoman passed an envelope to the judge. Jurors remained standing while the clerk read aloud the 24-page verdict form that they had filled out. It took 20 minutes.

It was not clear until the end that the sentence was death, though signs along the way pointed in that direction.

The jury took 14 hours to reach its sentence, which some legal specialists said was quick in a case this complex.

Eric Freedman, a death penalty specialist at Hofstra University Law School, said that the speed of the verdict suggested two grounds for an all-but-certain appeal that could last years: “the failure to grant a change of venue, despite the overwhelming evidence the defense presented about community attitudes in Boston, and the failure to instruct the jury that if a single juror refused to vote for death, the result would be a life sentence.”

“Unfortunately for all concerned,” he said, “this is only the first step on a long road.”

Other lawyers said that 14 hours was not speedy at all in a case like this, in which so much of the evidence weighed against the defendant, and they saw no serious grounds for appeal.

“I’ve seen juries return verdicts in 25 minutes if the evidence is strong,” said Michael Kendall, a former federal prosecutor in Boston. “But rarely do you have a case like this — a crime of such enormity to start with, plus a mountain of evidence and a defendant who is so unsympathetic.”

He said he thought Tsarnaev’s callousness had struck the jury. “After he blows up this child on purpose, he’s out at the convenience store buying milk, then he smokes a little dope and plans on blowing up New York,” Kendall said.

Tsarnaev stood, his hands folded in front of him, as the jurors made their way out of the courtroom. Moments earlier, the judge, George A. O’Toole Jr. of U.S. District Court, thanked them and sent them on their way: “So jurors, that’s it.”

It was the first time a federal jury had sentenced a terrorist to death in the post-Sept. 11 era, according to Kevin McNally, director of the Federal Death Penalty Resource Counsel Project, which coordinates the defense in capital punishment cases.

For many in the Boston area, the sentencing brought “a small amount of closure to the survivors, families and all impacted by the violent and tragic events,” Mayor Martin Walsh said in a statement that did not explicitly praise the verdict. “We will forever remember and honor those who lost their lives and were affected by those senseless acts of violence on our city.”

Attorney General Loretta Lynch called the death sentence a “fitting punishment.”

In Russia, when contacted by a reporter and informed of the verdict, Tsarnaev’s father, Anzor, simply exhaled and hung up. He then turned off his cellphone.