Army Sgt. La David Johnson, one of the four American soldiers killed in the ambush in Niger last month, may have been captured by Islamic militants prior to his death, a report Friday said.

Nigerian military sources said they think fighters linked to the Islamic State decided to capture and kidnap Johnson, according to CBS News. Hands tied, he was later shot and his body disposed in a bush.

The Pentagon has not disclosed how Johnson, who was found two days later about a half mile away from the ambush, was separated from other soldiers.

The soldiers were attacked as they were returning to their operating base following a visit to a village, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said last week.

He added that air support had been requested approximately an hour after the attack and probably hadn't been called earlier because soldiers thought they didn't need it.

The U.S. has approximately 800 troops in Niger and 6,000 conducting missions throughout Africa. At least 20 Americans have lost their lives serving the nation since January.