(Deletes reference to nitrous oxide as a 'highly flammable gas' in fifth paragraph)

By Dan Whitcomb

LOS ANGELES, July 27 (Reuters) - Three people were killed and three others critically injured on Thursday in an explosion at the California rocket-testing facilities operated by famed commercial spaceflight pioneer Burt Rutan, authorities said.

The blast occurred at the Mojave Air and Space Port, where Scaled Composites, the company formed by Rutan, was testing a rocket motor, Kern County Fire spokesman Tony Diffenbaugh said.

He said two people were killed instantly in the explosion at 2:34 p.m. PDT (2134 GMT) and four others were rushed to a local hospital. One of those died following surgery, according to the Los Angeles Times. The sprawling, 3,300-acre (1,335-hectare) airfield complex was immediately shut down.

"Our units arrived on the scene at a remote test site in the northeast portion of the airport. What they found was six victims of an apparent explosion with various traumatic and burn injuries," Diffenbaugh said.

Television images showed wrecked equipment and vehicles and blackened ground from the blast, which Diffenbaugh said involved nitrous oxide.

Fire crews waited several hours before approaching the scene because of the extreme danger.

"We just don't know" why the explosion occurred, Rutan said at a news conference. He noted that nitrous oxide usually is "not considered a hazardous material."

Rutan, who has partnered with Microsoft billionaire Paul Allen and British airline tycoon Richard Branson to form a space tourism company, largely bases his company, Scaled Composites, at the airfield.

In 2004 Rutan's SpaceShipOne, a stubby, three-seat rocket plane, made the world's first privately funded manned spaceflight.

The Mojave Airport opened in 1935 as a small rural airfield serving the local gold and silver mining industry and was taken over by the U.S. Marine Corps in 1942 as a military base.

It was returned to civilian use in 1961 and is currently home to the National Test Pilot School, the largest civilian test pilot center in the world.

It also houses one of the country's largest aviation scrap yards and has been used as a location in numerous TV shows and films, including "Die Hard 2" and "Waterworld." (Additional reporting by Steve Gorman and Mary Milliken)

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