The US military has lost contact with the unmanned hypersonic glider that was launched for a test flight over the Pacific last week.
The Falcon hypersonic technology vehicle-2 was supposed to separate from its Minotaur 4 rocket and glide at 13,000mph before splashing down in the Kwajalein Atoll, 2,500 miles southwest of Hawaii.
A Defence Advanced Research Projects Agency (DARPA) spokesman told the Associated Press that the separation did not occur.
Contact with the glider was lost nine minutes after lift-off, but according to preliminary reports it achieved controlled atmospheric flight before telemetry was lost.
Data from the US Air Force, Navy, Army and Missile Defence Agency is being analysed to determine what happened.
According to DARPA, the vehicle was never supposed to be recovered and the test would have been terminated after it achieved 30 minutes of flight.
The mission was designed to test flight technologies at high speed with the US military trying to develop a craft able to respond to global threats at speeds of more than Mach 20.
The glider was developed by Lockheed Martin and the Minotaur rocket was created by Orbital Science Corp from decommissioned Peacekeeper ballistic missiles.