BOSTON — A federal appeals court on Thursday ordered the judge who oversaw Boston Marathon bomber Dzhokhar Tsarnaev’s trial to investigate the defense’s claims of juror bias and determine whether his death sentence should stand.

A three-judge panel of the Boston-based 1st U.S. Circuit Court of Appeals did not throw out Tsarnaev’s death sentence. Defense lawyers had pushed for that while claiming bias by two people who sat on the jury that convicted him for his role in the bombing that killed three people and injured hundreds near the marathon’s finish line in 2013.

But the appeals court found that the trial judge did not adequately probe Tsarnaev’s allegations, and sent the case back to the judge for a new investigation. If the judge finds that either juror should have been disqualified, he should vacate Tsarnaev’s sentence and hold a new penalty-phase trial to determine whether Tsarnaev should be sentenced to death, the appeals court said.

“And even then, we once again emphasize that the only question in any such proceeding will be whether Tsarnaev will face execution; regardless of the outcome, he will spend the rest of his life in prison,” it said.

The U.S. attorney’s office in Massachusetts declined to comment Thursday. The Justice Department can either ask the full 1st Circuit to hear the matter or go to the U.S. Supreme Court. Lawyers for Tsarnaev didn’t immediately respond to emails seeking comment on the decision.

It’s the latest twist in the long-running case, which has already been argued in front of the U.S. Supreme Court once. The high court in 2022 reinstated the death sentence imposed on 30-year-old Tsarnaev after the 1st Circuit threw out the sentence in 2020. The circuit court found then that the trial judge did not sufficiently question jurors about their exposure to extensive news coverage of the bombing. The Supreme Court justices voted 6-3 in 2022 when they ruled that the 1st Circuit’s decision was wrong.

The 1st Circuit took another look at the case after Tsarnaev’s lawyers urged it to examine issues the Supreme Court didn’t consider. Among them was whether the trial judge wrongly forced the trial to be held in Boston and wrongly denied defense challenges to seating two jurors they say lied during questioning.

Despite a moratorium on federal executions imposed by Attorney General Merrick Garland, the Justice Department has continued to push to uphold the death sentence in Tsarnaev’s case. The moratorium came after former President Donald Trump’s administration put to death 13 inmates in its final six months.

Oral arguments before the three-judge 1st Circuit panel more than a year ago focused on two jurors Tsarnaev’s lawyers say were dishonest during the lengthy jury selection process.

One of them said she had not commented about the case online, but she had retweeted a post calling Tsarnaev a “piece of garbage.” Another juror said none of his Facebook friends had commented on the trial, even though one had urged him to “play the part” so he could get on the jury and send Tsarnaev to “jail where he will be taken care of,” defense attorneys say. Tsarnaev’s lawyers raised those concerns during jury selection, but say the judge chose not to look into them further.

William Glaser, a Justice Department lawyer, acknowledged during oral arguments before the 1st Circuit appeals court that the jurors made inaccurate statements, but said other disclosures suggested they misremembered rather than lied. He argued that the trial judge did nothing wrong.

The appeals court said there are potentially “innocuous” explanations for the jurors’ conduct, like they forgot about their social media posts or misunderstood the judge’s questions. But the trial judge’s “error was in failing to conduct an inquiry sufficient to rule out the more pernicious explanations,” the appeals court said.

The appeals court panel voted 2-1 in favor of sending the case back down for more investigation into the jurors. Those who supported the idea were Judges William Kayatta Jr. and O. Rogeriee Thompson, who were both nominated to the court by President Barack Obama.

Judge Jeffrey Howard, who was nominated by George W. Bush, dissented, writing that there was “ample basis for the district court to arrive at the judgment that the two jurors in question were not improperly biased.”

Tsarnaev’s guilt in the deaths of Lingzi Lu, a 23-year-old Boston University graduate student from China; Krystle Campbell, a 29-year-old restaurant manager from Medford, Massachusetts; and 8-year-old Martin Richard, of Boston, was not at issue in the appeal, only whether he should be put to death or imprisoned for life.

Defense lawyers argued that Tsarnaev had fallen under the influence of his older brother, Tamerlan, who died in a gun battle with police a few days after the April 15, 2013, bombing.

Tsarnaev was convicted of all 30 charges against him, including conspiracy and use of a weapon of mass destruction and the killing of Massachusetts Institute of Technology Police Officer Sean Collier during the Tsarnaev brothers’ getaway attempt.

Prosecutors told jurors that the men carried out the attack to punish the United States 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.”