Mojave, Calif. ? An explosion Thursday 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 at 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.

It was not immediately known what exploded or how it happened. 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 said.

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.

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.