In a 6-3 decision, the Supreme Court reinstated the death penalty on Dzhokhar Tsarnaev, the Boston Marathon bomber, reversing a lower court's decision that vacated his death sentence.

The high court's decision was split along ideological lines, with all six conservative justices agreeing with the Biden administration in its argument that the First U.S. Circuit Court of Appeals in Boston was wrong to throw out the death sentence.

"Dzhokhar Tsarnaev committed heinous crimes," Justice Clarence Thomas wrote in the opinion of the court. "The Sixth Amendment nonetheless guaranteed him a fair trial before an impartial jury. He received one."

Tsarnaev was convicted in 2015 for his role in the bombing, which killed three people and injured nearly 300 more. A jury found Tsarnaev guilty of joining his brother, Tamerlan, in planting and detonating two pressure cooker bombs near the finish line of the world's oldest annual marathon in 2013. 

After the bombing, the brothers fled; in the process, they killed Massachusetts Institute of Technology Police Officer Sean Collier. They later engaged in a shootout with police, and Dzhokar inadvertently ran over Tamerlan with an SUV in his escape from authorities. Tamerlan was taken to a hospital, where he was later pronounced dead.

The federal appeals court ruled in 2020 that the trial judge improperly excluded evidence that could have shown Tsarnaev was deeply influenced by his older brother, Tamerlan, and was somehow less responsible for the carnage.

The appeals court also faulted the judge for not sufficiently questioning jurors about their exposure to extensive news coverage of the bombing.

"In my view, the Court of Appeals acted lawfully in holding that the District Court should have allowed Dzhokhar to introduce this evidence," Justice Stephen Breyer wrote in adissenting opinion.

"I have written elsewhere about the problems inherent in a system that allows for the imposition of the death penalty," Breyer added. "This case provides just one more example of some of those problems."

The Associated Press contributed to this report.