WASHINGTON — The U.S. military believes someone in a Niger village may have tipped off attackers to the presence of U.S. commandoes and Nigerien forces in the area, setting in motion the ambush that killed four Americans, a senior defense official said Tuesday.

The official said the Army Green Berets and about 30 Niger forces stopped in a village for an hour or two to get food and water after conducting an overnight reconnaissance mission. After they left, they were ambushed by about 50 heavily armed enemy fighters, who also killed four Niger fighters and wounded two Americans and several Niger troops.