An unmanned experimental aircraft designed to fly six times the speed of sound has broken apart over the Pacific Ocean seconds into a US military test flight.
The US Air Force later said it was due to a faulty control fin.
The Waverider was designed to reach speeds of Mach 6 or above, fast enough to zoom from New York to London in less than an hour.
Analysts say the US wants to use it to develop missiles with non-nuclear explosives that could strike anywhere in the world within an hour.
Guy Ben-Ari of the Center for Strategic and International Studies said if the programme becomes operational, targets could include conventional military sites or militants.
Even if the test had been a success, the aircraft would have crashed at the end of the flight and was not considered retrievable.
This was the third of four X-51A aircraft built for the military.
One flew for over three minutes at nearly five times the speed of sound during a 2010 test flight, the Air Force said in a statement.
The Air Force said one X-51A aircraft remains and that a decision has not been made "when or if that vehicle will fly at this time".
The cost of the experimental aircraft has not been disclosed because many details of the programme are classified.
The aircraft is known as the Waverider because it stays airborne, in part, with lift generated by the shock waves of its own flight.
The Boeing Co's Phantom Works division performed design and assembly on the craft, the military said.
A Boeing spokeswoman declined to comment on the test flight, citing an Air Force request to have all public communication come from the military.
The problem with the fin on the craft was identified in a test flight on Tuesday, 16 seconds after a rocket booster on the remotely monitored craft was ignited to propel it forward.
Then 15 seconds later, when the X-51A separated from the rocket booster, it lost control due to a "faulty control fin".
The 31 seconds of flight fell far short of the military's goal for the X-51A to fly for five minutes.
The aircraft broke apart immediately and fell into the Pacific Ocean near Point Mugu northwest of Los Angeles, said Daryl Mayer, a spokesman for the 88th Air Base Wing at Wright-Patterson Air Force Base in Ohio.
The fins on the rocket booster kept the aircraft on-course during the initial phase of the flight, despite the problem with the control fin on the X-51A itself, Mr Mayer said.
