A private rocket carrying three satellites, including a Nasa space sail, has failed to reach orbit.
It is the third unsuccessful launch attempt for the Falcon 1 rocket, built by private space firm SpaceX.
The vehicle's two stages failed to separate two minutes and 20 seconds into launch from Kwajalein Atoll in the Pacific Ocean.
SpaceX's engineers are still investigating the cause of the rocket's failure.
"It was obviously a big disappointment not to reach orbit on this Flight 3 of the Falcon 1," said Elon Musk, chief executive of SpaceX, in a short statement read to reporters.
"The most important message I'd like to send right now is that SpaceX will not skip a beat in execution going forward. We have flight four of Falcon 1 almost ready for flight and flight five right behind that."
Mission gap
Mr Musk has invested millions of dollars earned from the sale of his two internet financial services firms into SpaceX to develop low-cost space transportation.
The Falcon 1 rocket was carrying Trailblazer, a small satellite built by the Pentagon, as well as a mini-laboratory called PreSat and a 10-sq-m "solar sail" - both of which were built by the US space agency (Nasa).
The incident comes on the heels of two Falcon 1 launch attempts that fell short of orbit.
The first launch in March 2006 ended with a fuel line leak and a fire caused by a corroded nut.
During the second attempt in March 2007, the rocket reached 290km (180 miles) above Earth and nearly achieved orbital velocity before the vehicle's second stage engine shut down because of fuel slosh and roll-control issues.
On Saturday, SpaceX also fired its Falcon 9 rocket, designed to ferry cargo and astronauts to the International Space Station (ISS).
The demonstration in Texas meets a target laid down by Nasa, which has given SpaceX some "seed" funding.
Nasa hopes the Falcon 9 can help fill the mission gap that will exist while it develops a successor to the shuttle.
Mr Musk commented: "As a precautionary measure to guard against the possibility of flight three not reaching orbit, SpaceX recently accepted a significant investment.
"Combined with our existing cash reserves, that ensures we will have more than sufficient funding on hand to continue launching Falcon 1 and develop Falcon 9 and Dragon."