The U.S. 1st Circuit Court of Appeals in Boston ordered a lower court to hold a new trial on the issue of what penalty Tsarnaev should receive. It said the trial court did not hold proper juror inquiries about the impact of pretrial publicity.
The appeals court upheld much of his conviction for the 2013 attack, which killed three people and wounded more than 260 others.
Tsarnaev and his older brother, Tamerlan, radical Islamist Jihadists, detonated two homemade bombs near the finish line of the historic race, causing that the court said was "battlefield-like carnage." They also killed local campus police officer Sean Collier later.
Tamerlan was killed during a shootout with police several days after the bombing.
The First Circuit Court said the trial judge had to ascertain not just the degree but the "kind" of exposure that prospective jurors had to publicity about the case.
"But as to 9 of the 12 seated jurors, the judge fell short on this front," the court said.
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