More than 400 large U.S. military drones have crashed in accidents around the world since 2001, according to an investigation by The Washington Post, highlighting the dangers posed by the technology.

A key weapon in the war on terror, drones look set to become part of America’s airspace. Thanks to a law passed in 2012, commercial and law enforcement use of drones is expected to increase significantly over the coming years.

The Post’s year-long investigation, however, raises questions about drone safety, identifying 418 major crashes involving U.S. military drones between September 2001 and the end of 2013.

According to Air Force safety statistics, the drone crash total is almost equivalent to the number of crashes involving Air Force fighter jets and attack planes over the same period. The drones, however, flew far fewer hours and missions, according to the statistics.