The FAA is significantly behind schedule in drawing up rules and standards to ensure that drones are airworthy, that pilots are trained properly and that their aircraft won't interfere with other air traffic, the Transportation Department's inspector general concluded in a highly critical audit.

The report said the FAA has so many hurdles to confront that it is unclear when — or if — remotely controlled aircraft can be safely integrated into U.S. airspace.

In 2012, Congress passed a law that effectively legalized commercial and civilian drone flights in the United States. With backing from drone manufacturers and other businesses eager to embrace a new era in aviation, lawmakers directed the FAA to come up with rules and standards to permit widespread drone traffic by Sept. 30, 2015.