The FBI has joined the investigation into how a group of ISIS-affiliated militants killed four U.S. soldiers in Niger on Oct. 4, the Wall Street Journal reported.
Military officials said they're still trying to piece together a timeline of what happened amid lawmakers' criticism of the Pentagon and White House for a lack of transparency, the Journal reported.
The involvement of the FBI in the probe of a military operation isn't unprecedented; the agency has the authority to take over the investigation but hasn't yet done so, the Journal reported, citing unnamed officials.
FBI investigators are helping gather and evaluate evidence about the militants considered responsible for the ambush, and how they learned of the joint U.S.-Nigerien patrol.
"These four soldiers being killed and most people not knowing what they were up to is a game changer," said Sen. Lindsey Graham, R., S.C., a member of the Senate Armed Services Committee, the Journal reported. "I'm concerned that we're not regularly briefed about operations."
Committee chair, Sen. John McCain, R., Ariz., said he hadn't been properly informed by the Pentagon about the mission – and is threatening to use legislative measures to force compliance with demands for information, the Journal reported.