Admiral who ordered the follow-up attack on Sept. 2 that killed two men briefed lawmakers. The U.S. later said four ‘narco-terrorists’ were killed in a fresh strike on Thursday.

The new detail further complicates the military’s explanations for its actions during the Sept. 2 strike in the Caribbean Sea.

The arrest marks the first time investigators have settled on a suspect in an act that had long vexed law enforcement, spawned a multitude of conspiracy theories and remained an enduring mystery in th...