A senior congressional aide who has been briefed on the deaths of four U.S. servicemen in Niger says the ambush by militants stemmed in part from a “massive intelligence failure.”

The Pentagon has said that 40 to 50 militants ambushed a 12-man U.S. force in Niger on Oct. 4, killing four and wounding two. The U.S. patrol was seen as routine and had been carried out nearly 30 times in the six months before the attack, the Pentagon has reported.

The aide, who spoke on condition of anonymity because he is not authorized to speak publicly, said the House and Senate armed services committees have questions about the scope of the U.S. mission in Niger, and whether the Pentagon is properly supporting the troops on the ground there.