MOJAVE, Calif. -- An explosion at the Mojave Airport killed three people and injured three others Thursday.

Two tanks of nitrous oxide exploded around 2:40 p.m. at a Scaled Composites rocket testing facility on the Mojave Airport premises, killing Scaled Composites employees Eric Blackwell, 38, of Randsburg and Charles May, 45, of Mojave instantly. A third person, Todd Ivens, 33, of Tehachapi, died at Kern Medical Center late Thursday evening, according to the coroner.

The other employees were airlifted to KMC, where two were in critical condition and one was in serious condition late Thursday.

"We don't know why it exploded," said Burt Rutan of Scaled Composites, who was emotional during a briefing with reporters.

Rutan says the workers were checking the rate nitrous oxide flowed through an orifice.

It's a test Scaled Composities performed many times before for SpaceShipOne, and were conducting this test for components of its successor.

Nitrous oxide serves as an oxidizer to help rocket fuel burn.

The explosion is a setback for the company that sent the first private manned aircraft to space, and is working with Richard Branson's Virgin Galactic company towards space tourism.