An explosion on 26 July has occurred near to Mojave airport involving aerospace company Scaled Composites employees.
The number of deaths or injured people at the scene were reported to have been up to three dead and at least four injured.
Scaled Composites chief executive Burt Rutan said, according to news reports, that the blast, which released nitrous oxide into the air, did not involve a rocket firing.
The explosion happened during a test of the flow of nitrous oxide through an injector in the course of testing components.
These are for a new rocket motor for the proposed suborbital rocket-glider SpaceShipTwo (SS2), which is being developed for Sir Richard Branson's Virgin Galactic space tourism company.
Rutan gave little additional information about the test, but said it had been done safely many times during the SpaceShipOne programme and had been done once before for the SS2 programme.
"We were doing a test we believe was safe. We don't know why it exploded. We just don't know," he said.
Speaking to flightglobal.com, Virgin Galactic president Will Whitehorn confirmed the Scaled work was related to SpaceShipTwo.
He said: "Our deepest sympathies are with the families. It didn’t involve the [SpaceShipTwo] rocket motor.
"We won't comment further until the results of the investigation by the relevant authorities. This will delay [the development programme]."