The employees who were killed in a blast last year at a California spaceship company were watching a propulsion system test though a chain-link fence when the explosion occurred, according to a report from the California Division of Occupational Safety and Health.

Three employees of Scaled Composites were killed in July after a “cold flow” test of the nitrous oxide gas used to propel the spacecraft failed three seconds into the procedure. The test did not involve igniting the engine. The exact cause of the accident is under review.

According to Cal/OSHA report, one group of workers conducting the test moved to safety in a vehicle 430 feet away and tucked behind a 15-foot-high earthen berm, and then called a second group of workers to tell them the test was about to begin. Eleven of those workers gathered at the chain-link fence near the test equipment to watch. Along with the three who died, three other employees were critically injured.

The report concluded, “The employer did not provide the workers effective information and training on the health and physical hazards of the substances in the work area” and did not train supervisors to deal with those hazards.

Last month, the agency issued five workplace citations to Scaled Composites and fined the company $28,870. The company has appealed.

The nitrous oxide used with the rubber-based fuel at Scaled Composites had been, until the accident, considered one of the safest fuels in rocketry, according to a New York Times article. The Federal Aviation Administration in 2004 declared the work of Scaled Composites at the Mojave airport “benign to the public,” and the motor was declared “not explosive” and “extremely difficult to ignite accidentally.”