Secretary of Defense James Mattis met with Sens. John McCain and Lindsey Graham on Friday after the ambush in Niger more than two weeks ago that resulted in the death of four American soldiers, according to CBS.

Why it matters: There are still several unanswered questions about the ambush, and the FBI has joined the investigation. Sen. McCain said on Thursday that the investigation may "require a subpoena," but Sec. Mattis maintained that didn't prompt their meeting.