James Mattis meets with John McCain amid questions about Niger ambush

Defense Secretary James Mattis met with Sen. John McCain on Friday afternoon on Capitol Hill in the wake of growing questions about what happened in the Niger ambush earlier this month that resulted in the deaths of four U.S. soldiers.

McCain, the chairman of the Senate Armed Services Committee, said Thursday that the investigation involving the Niger attack could "require a subpoena."

"It may require a subpoena but I did have a good conversation with General McMaster and they said they would be briefing us. We have a long friendship and we'll hopefully get all the details," McCain said.

When asked if the threat of the subpoena prompted the meeting Friday, Mattis responded, "Are you kidding me?"

After their meeting, Mattis went to the office of Sen. Lindsey Graham, R-South Carolina.

A U.S. law enforcement official confirmed to CBS News on Friday that the FBI is now assisting the military in its investigation of the ambush in Niger. The Wall Street Journal first reported the FBI's involvement. The FBI has been involved in other recent ambushes of U.S. military personnel overseas including last year's attack on soldiers in Jordan.

CBS News' David Martin reported Thursday. on new details that emerged regarding what happened in the ambush.