One of the four U.S. soldiers who was killed in a deadly ambush in Niger last month may have been captured by Islamic militants, Nigerien military sources told CBS News.

Army Sgt. La David Johnson's body was found two days after roughly a dozen U.S. soldiers and 40 Nigerien troops were ambushed by militants during a joint patrol in the African country.

CNN reported last month that Johnson's body was found a mile away from the site of the attack. His hands were bound with rope.

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The delay in recovering Johnson's body has sparked questions among lawmakers and military officials.

While the Pentagon has launched a formal investigation into the deadly incident, it has not yet said how Johnson became separated from his comrades.

"The loss of our troops is under investigation," Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE told reporters. "We investigate anytime we have our troops killed, whether it be in a training accident or combat."

Johnson was laid to rest in Florida last month.

Johnson and his family became the subject of national debate after Rep. Frederica Wilson Frederica Patricia WilsonHarris calls it 'outrageous' Trump downplayed coronavirus House passes bill establishing commission to study racial disparities affecting Black men, boys Florida county official apologizes for social media post invoking Hitler MORE (D-Fla.) revealed to various news outlets that she overheard President Trump tell his widow that her late husband “knew what he signed up for ... but when it happens it hurts anyway.”

The White House has repeatedly disputed Wilson's account of the phone call, which was backed up by Johnson's family.