WASHINGTON (AP) — Federal safety investigators said Tuesday the crash of a Virgin Galactic spaceship last year was caused by a catastrophic structural failure triggered when the co-pilot unlocked the craft's braking system early.

National Transportation Safety Board investigators said the resulting aerodynamic forces caused the brakes to actually be applied without any further action by the crew. Investigators said no safeguards were built into system to overcome the error of the co-pilot.

The spaceship broke apart over the Mojave Desert during a test flight 10 months ago. The accident killed the co-pilot and seriously injured the pilot.

NTSB officials said early in the investigation that the co-pilot prematurely unlocked equipment designed to slow the descent of the spacecraft during initial re-entry. Simply unlocking the spacecraft's brakes shouldn't have applied them, but investigators had said that might have happened anyway and the resulting stress may have contributed to the spacecraft's destruction.

NTSB chairman Christopher Hart said he hoped the investigation will prevent such an accident from happening again. He said the NTSB learned "with a high degree of certainty the events that resulted in the breakup."

"Many of the safety issues that we will hear about today arose not from the novelty of a space launch test flight, but from human factors that were already known elsewhere in transportation," Hart said.

Scott Pace, director of the Space Policy Institute at George Washington University, said the industry will be looking to see if the NTSB goes beyond the specific cause of the accident in its findings.

"Simply focusing on an immediate cause is usually not enough to understand deeply how to improve safety," Pace said.

Virgin Galactic has been proceeding with its plans for space flight and is now building another craft. Company officials have said in recent months that their commitment to commercial spacecraft has not waivered despite the crash and they expect the company to resume test flights later this year. Eventually, the company envisions flights with six passengers climbing more than 62 miles above Earth.