Three people were killed and three others seriously injured in an explosion at the California rocket-testing facilities operated by commercial spaceflight pioneer Burt Rutan.
The blast occurred at the Mojave Air and Space Port, where Scaled Composites, the company formed by Mr Rutan, was testing a rocket motor.
Two people were killed instantly in the explosion yesterday afternoon, and another person died following surgery. The 3,300-acre complex was immediately shut down.
The explosion involved the highly flammable gas nitrous oxide, and fire crews waited several hours before approaching the scene.
Mr Rutan said the blast happened during a test of the flow of nitrous oxide through an injector while testing components for a new rocket motor for the upcoming SpaceShipTwo.
The nitrous oxide was at room temperature and under pressure. "We were doing a test we believe was safe. We don't know why it exploded. We just don't know," he said.
Mr Rutan has joined with Microsoft billionaire Paul Allen and British airline tycoon Sir Richard Branson to form a space tourism company. In 2004 Mr Rutan's three-seater SpaceShipOnemade the world's first privately funded manned spaceflight.