A record number of U.S. Air Force MQ-9 Reaper unmanned aerial vehicles crashed in 2015 due to electrical problems, even as the drones are in high demand.

The General Atomics MQ-9 Reaper, previously called the Predator B, which followed the MQ-1 Predator, is used in a variety of missions by the U.S. Armed Forces and its allies, which were cleared to purchase the aircraft from the Pentagon by the U.S. State Department in February 2015. These missions include intelligence, surveillance and reconnaissance operations, as well as counterterrorism operations against the Islamic State, Sunni militants based in Iraq and Syria also identified as Daesh and the acronyms ISIS and ISIL.

The Washington Post reports 20 Reaper aircraft were destroyed or sustained at least $2 million in damage throughout 2015, an all-time annual high for the program. Documents obtained by the Post under the Freedom of Information Act reveal many of the aircraft experienced sudden electrical failures causing them to fall from the sky.

Investigators say the problem can be traced to a faulty starter-generator, however a permanent fix has yet to be found.

The report comes as many U.S. allies have long-term procurement plans for the Reaper. The French military plans to have a fleet of 12 of the aircraft by 2019, potentially with more to follow, and Italy is in the process of integrating a variety of weapons on their own Reaper fleet.

The State Department also approved the sale of four MQ-9s to Spain in October 2015, as U.K. Prime Minister David Cameron announced his intent to replace his country's fleet of 10 Reapers with 20 newer Reapers, dubbed Protector, though it's not clear what configuration they will be.

A MQ-9 Reaper unit comes at the cost of over $64 million per unit, which includes four aircraft, sensors and supporting equipment.