According to a new investigation from The Washington Post, the Air Force had its worst year for drone crashes ever in 2015, as its Reaper drone was suddenly plagued by crashes.

The Post found that electrical failures have been causing the massive military drone to suddenly drop out of the sky. Although investigators have traced the problems back to issues with the drones' "starter-generator," the exact reason for the problems, and the solution, haven't been determined.

Meanwhile, the drones keep falling. The 20 large Air Force drones that crashed last year caused $2 million in damage, the Post reported, although no one was killed by a crash. Ultimately, 10 Reaper drones were damaged or destroyed, a figure that was at least twice as high as any other year, according to the Post. Half of the total drone crashes were not reported by the Department of Defense, despite a policy to disclose major mishaps.

The Air Force has already been publicly bemoaning a lack of funding and people. Last month, it announced a proposal to hire thousands of new personnel for its drone program.