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Details are scant at this time, but a explosion at the Scaled Composites rocket test facility has killed two people and seriously injured four more. The Los Angeles Times reports that the explosion was "ignited by a tank of nitrous oxide".

This is Burt Rutan's facility, and the home of SpaceShip One and Virgin Galactic spacecraft development.
http://www.latimes.com/news/local/la-me-explode27jul27,0,3190584.story?coll=la-home-local