The U.S. Army has seen a recent “spike” in unmanned aircraft mishaps and is trying to figure out what caused the increase, according to a service official.

The Army has experienced eight unmanned aviation accidents so far in fiscal year 2016, which began Oct. 1, said Lt. Gen. Kevin Mangum, deputy commanding general of U.S. Army Training and Doctrine Command (TRADOC). “Those are still under investigation to try to determine what the root cause of those” is, Mangum said.

ArmyBy contrast, serious mishaps for manned aviation have declined during the same period, even though the Army lost six manned aircraft and eight aircrew members in the first six months of FY 2016. Investigators have attributed five of those six accidents to human error, said Mangum, who testified July 6 before the House Armed Services Committee’s readiness panel.

Mangum also testified that he is nearing completion of a “comprehensive assessment” of Army aviation, including readiness challenges. He said he has briefed Gen. Mark Milley, the Army’s chief of staff, on his team’s findings and plans to finalize a report “very soon.”