The fatal mission in Niger last year that left four American soldiers dead was not approved by senior command, a military investigation found.

The four members of a U.S. Army Special Forces team set out after Doundou Chefou, an Islamic State in Iraq and Syria militant suspected of involvement in the kidnapping of a U.S. aid worker, but did not inform their commanders, according to The Associated Press.

The soldiers were killed in a remote area more than 120 miles north of Niger's capital city in October. Nearly 100 Islamic State soldiers ambushed the Americans as they were leaving an area during the search for Chefou.

Two other American soldiers and eight Nigerien troops were wounded in the violent gun battle, which was partially recorded on one soldier's helmet camera.

Officials at U.S. Africa Command have been investigating the incident for months, under intense scrutiny from government officials and the media.