An explosion killed two workers and critically injured four others at a Mojave Desert airport site used by the pioneering aerospace company that sent the first private manned rocket into space, authorities said. The blast Thursday at a Mojave Air and Space Port facility belonging to Scaled Composites LLC also left some toxic material, said Kern County fire Capt. Doug Johnston. All the victims worked for Scaled. Scaled is the Mojave-based builder of SpaceShipOne, the first private manned rocket to reach space, and is developing a successor for the new space tourism business Virgin Galactic. Aerospace designer Burt Rutan, who heads Scaled, was away at the time. He sounded distraught in a phone call with The Associated Press as he was en route to the scene. "We've lost a couple of our employees. It's a very big deal," Rutan said. Scaled's offices and aircraft construction facilities were closed late Thursday. Authorities did not allow access to the blast site in a remote unpaved area about a quarter-mile beyond an airplane storage area. Video news helicopters showed wrecked equipment and vehicles at the airport in the high desert north of Los Angeles near Edwards Air Force Base. The accident involved nitrous oxide, but it was not known if an actual rocket motor test was under way or whether it occurred during preparation for a test, fire Inspector Tony Diffenbaugh told KABC-TV. Scaled uses nitrous oxide as an oxidizer in its rockets, which are tested at the airport. An oxidizer provides the oxygen that rocket fuel needs to burn. Scaled's Web site notes that "temperatures and pressures must be carefully controlled" during oxidizer transfers. Paramedics reported two people were killed, four were critically injured and one suffered minor injuries, said Mark Corum, a spokesman for Hall Ambulance Service. The injured were airlifted to Kern Medical Center about 70 kilometers from the airport, he said. A call seeking comment from the airport manager was not immediately returned. The Mojave airport is where the Rutan-designed Voyager aircraft was built. It made history in 1986 when it achieved the first nonstop flight around the world without refueling. In 2004, Rutan's SpaceShipOne, funded by Microsoft co-founder Paul Allen, made the first privately financed manned spaceflight by climbing more than 100 kilometers high on a suborbital journey above Mojave. SpaceShipOne went on to make two more flights to win the $10 million Ansari X Prize. Rutan has since been developing SpaceShipTwo for entrepreneur Richard Branson's Virgin Galactic, which plans to offer $200,000 rides into space for tourists. Branson is investing at least $200 million for a fleet of Rutan's spaceships. Earlier this year he told a trade show the new ship will be ready within a year and, after a year of flight tests, would have its first commercial launch in 2009. Rutan, who works secretively and stresses that safety will be the key to success of space tourism, has not publicly released a schedule for completion of the design, testing and first launch. Aerospace and defense contractor Northrop Grumman Corp. owns 40 percent of Scaled and recently agreed to acquire the rest of it. The deal is awaiting regulatory approval and should close next month. Northrop Grumman spokesman Dan McClain said the company had no comment on the explosion.