An emerging theory among U.S. military investigators is that the Army Special Forces soldiers ambushed in Niger were set up by terrorists, who were tipped off in advance about a meeting in a village sympathetic to local ISIS affiliates, three U.S. officials who have been briefed on the matter told NBC News.

The group of American Green Berets and support soldiers had requested a meeting with elders of a village that was seen as supportive of ISIS, and they attended the meeting at around 11 a.m. local time on Oct. 4, after a long night of patrolling, the officials said. Such meetings are a routine part of the Green Beret mission, but it wasn’t clear whether this meeting was part of the unit’s plan.