Six U.S. troops died Tuesday, after a helicopter crash in Afghanistan. The Pentagon is investigating whether they may have been killed by enemy fire after the crash.

Initial reports cited potential mechanical failure and that there was no indication of enemy activity in the area. But two senior U.S. officials say there are questions about whether the troops may have actually survived the crash and then came under motor fire.

One person on board the aircraft survived. Investigators are looking at the wreckage and will conduct autopsies for clues. This was the single-deadliest day for U.S. forces in Afghanistan since August 2012.