First came the thunderous boom. Then the air billowed with thick black smoke. Fires were “everywhere,” survivors said. Scores of people lay dead.

The Nigerian military, with whom the U.S. military is fighting a growing Islamist threat, initially declared the attack on a weekend market in the remote desert village of Jilli a successful strike “on a known terrorist enclave.” But eyewitnesses describe a starkly different scene: The more than 100 people killed, they say, were traders and other members of the community, and included women and children.