Tsarnaev sentenced to death for Boston Marathon bombing

Convicted Boston Marathon bomber Dzhokhar Tsarnaev was sentenced to death Friday after a Boston jury's third day of deliberations.

Tsarnaev showed little expression as he heard the sentence, and the courtroom was solemn throughout the proceedings.

The 21-year-old was convicted last month of killing three people and injuring 264 others by detonating homemade bombs at the Boston Marathon finish line on April 15, 2013. Three days later, he and his 26-year-old brother shot a police officer to death, carjacked a Chinese businessman and hurled bombs at police, triggering a day-long lockdown of most of the Boston area as police searched for Tsarnaev.

Read MoreBoston Marathon bomber Tsarnaev guilty on all counts

Tsarnaev will remain in custody, with a formal sentencing in the coming months. He will become the youngest person on federal death row.

An appeals process could take years before Tsarnaev is killed by lethal injection.

A sketch of Dzhokhar Tsarnaev (right) in court May 15, 2015.
Art Lien | Courtartist.com
A sketch of Dzhokhar Tsarnaev (right) in court May 15, 2015.

U.S. District Judge George O'Toole called his jurors a "model for future juries." Some of the jurors cried as O'Toole addressed them.

During the trial, the jury saw gruesome, sometimes graphic videos of the explosions and their bloody aftermath, and heard from some of the 18 people who lost limbs in the bombing, as well as friends and family of the four people killed by the Tsarnaevs.

"The ultimate penalty is a fitting punishment for this horrific crime, and we hope that the completion of this prosecution will bring some measure of closure to the victims and their families," said U.S. Attorney General Loretta Lynch in a statement Friday.

In this courtroom sketch, Dzhokhar Tsarnaev, center, is depicted with defense attorney Miriam Conrad, right, as the verdict is read by the clerk during his federal death penalty trial, May 15, 2015.
Art Lien | Courtartist.com
In this courtroom sketch, Dzhokhar Tsarnaev, center, is depicted with defense attorney Miriam Conrad, right, as the verdict is read by the clerk during his federal death penalty trial, May 15, 2015.

The same jury last month found Tsarnaev guilty of all 30 counts against him related to the bombing. Seventeen of those qualified him for the death penalty.

Tsarnaev's lawyers had framed him as a hapless teenager under the influence of his radicalized older brother, Tamerlan, who was killed days after the bombing following a firefight with police.

Some people affected by the bombing—including the parents of 8-year-old victim Martin Richard—had publicly called for a sentence of life in prison. Richard's parents were present Friday and showed little emotion as the sentence was read.

Sydney Corcoran, who was injured in the bombing along with her mother, expressed relief at the sentence.

This story is developing. Please check back for further updates.

— Reuters and NBC News contibuted to this report