In Wednesday’s U.S. Supreme Court arguments in the case involving the Boston Marathon bomber, Justice Elena Kagan clashed briefly with Justice Brett Kavanaugh over his characterization of her questions to the government’s attorney.

The tense moment came as the justices focused on what appeared to be the most critical issue in the high court’s review of the decision by the U.S. Court of Appeals for the First Circuit to vacate Dzhokhar Tsarnaev’s death sentence. The appellate court ruled the trial judge erred in excluding mitigating evidence that Tsarnaev’s older brother, who also committed the bombings, was allegedly involved in three jihad murders two years earlier. Tsarnaev wanted that evidence to show he was under the influence of his violent brother, Tamerlan, who he claimed was the mastermind of the bombings.