Investigation of the accident at Scaled Composites in California on 26 July 2007 suggests the oxidiser tank for the propulsion system under development for Virgin Galactic's SpaceShipTwo suborbital rocket was the origin of the nitrous oxide explosion that killed three employees.

The explosion occurred at the Rocky propulsion test site, which Scaled rents from the Mojave air and space port. The summary report on the investigation by California's division of occupation health and safety (Cal-OSHA) describes the catastrophic event  as an "explosion while conducting rocket motor testing". It also details the events leading up to the explosion.

"The workers had transferred the nitrous oxide from the [mobile conditioning system] into the propulsion equipment apparatus tank mounted on the test stand. The catastrophic explosion occured about 3s after the [cold nitrous oxide] flow began," says the summary, published on 7 February.

Cal-OSHA is preparing to submit the investigation findings to the Kern county district attorney's office for a decision on whether Scaled will face prosecution. Such a decision is months away.

The Californian safety division has already fined the company more than $25,000 for not properly training its staff, which was deemed a contributor to the accident. Scaled filed notices of appeal against all the fines on 5 February, according to Cal-OSHA.