A U.S. appeals court overturned the death sentence for Boston Marathon bomber Dzhokhar Tsarnaev on Friday and ordered a lower court to hold a new penalty-phase trial.

Kyrgyz-American, 27, was sentenced to death in 2015 for planting two home-made bombs near the finish line of the race in 2013, killing three people and injuring more than 260 others.

He has admitted carrying out the attack as a 19-year-old with his older brother, Tamerlan Tsarnaev, who died four days after the atrocity in a gunfight with police.

Judge: Tsarnaev will remain in prison, pending a new trial

The federal appeals court in Massachusetts upheld most of Tsarnaev's convictions but instructed a district court to hold a new penalty-phase trial to determine his fate for crimes that carried the death sentence.

"Just to be crystal clear... Dzhokhar will remain confined to prison for the rest of his life, with the only question remaining being whether the government will end his life by executing him," the three judges said.

All three judges agreed the death sentence should be tossed.

Judge O. Rogeriee Thompson wrote that a presiding judge "fell short" in ensuring that a fair jury was selected, noting that some jurors had "already formed an opinion he was guilty."

Prosecutors: Tsarnaev carried out the attack to punish the U.S. for its wars in Muslim countries

A spokesperson for the U.S. Attorney's office in Boston said they were reviewing the opinion and had no immediate comment. Prosecutors could ask the full appeals court to hear the case or go straight to the U.S. Supreme Court.

Prosecutors told jurors that the men carried out the attack to punish the U.S. for its wars in Muslim countries. In the boat where Tsarnaev was found hiding, he had scrawled a confession that referred to the wars and wrote, among other things: "Stop killing our innocent people and we will stop."

Attorneys for Tsarnaev want the new trial out of Boston, question the neutrality of two jurors

Tsarnaev's attorneys had asked for a new trial, claiming that it should not have been held in Boston because the city was so traumatized by the attack.

They identified a slew of issues with his trial, but said in a brief filed with the court that the "first fundamental error" was the judge's refusal to move the case out of Boston.

They also questioned the neutrality of two jurors, pointing to social media posts from the jurors suggesting they harbored strong opinions even before the 2015 trial started.

Attorneys for Tsarnaev insisted "two jurors lied during jury selection about whether they'd had conversations about the case on social media."

'It's just terrible that he's allowed to live his life,' mother of a victim says

The mother of Krystle Campbell, the 29-year-old killed in the attack, expressed outrage at the court's decision.

"I just don't understand it," Patricia Campbell told The Boston Globe. "It's just terrible that he's allowed to live his life. It's unfair. He didn't wake up one morning and decide to do what he did. He planned it out. He did a vicious, ugly thing."

Former Massachusetts Bay Transportation Authority officer Dic Donohue, who was severely injured in a gunfight with the brothers, said the ruling was not surprising to him.

"And in any case, he won't be getting out and hasn't been able to harm anyone since he was captured," he tweeted.

(With input from agencies)