Niger says detainee is not mastermind of deadly attack: Sources Nigerien officials believed they had caught ISIS leader

Three senior Nigerien officials tell ABC News that they have determined that a man they have in custody is not Doundou Cheffou the ISIS leader whom officials believe masterminded the deadly ambush last October that killed four American soldiers.

ABC News had confirmed through senior Nigerien officials and a western official that Niger was trying to confirm the identity of a man captured several weeks ago along the border region with Mali.

Officials said the man in custody bore a resemblance to Cheffou, the head of ISIS in the Greater Sahara.

Senior Nigerien security officials, however, determined that the man was not the ISIS leader after testing his DNA and comparing it with Cheffou's brother who is in prison.

The New York Times was first to report on Monday that Nigerien officials were trying to determine if they had detained Cheffou.

Cheffou heads the small militant group known as ISIS in the Greater Sahara.

He is believed to have been responsible for the Oct. 4 ambush near the village of Tongo Tongo that killed four American soldiers.

The soldiers had been part of a joint U.S.-Nigerien patrol that had the night before been part of a mission to kill or capture Cheffou, who was known by his code name "Naylor Road".

U.S. Africa Command's investigation into the circumstances of that mission was completed in February.

After a review by General Joseph Dunford, the chairman of the Joint Chiefs of Staff, the investigation was referred to Defense Secretary Jim Mattis.

After Mattis has completed his review, family members, and Congress will be briefed on the report's conclusions.