Before their spaceship broke apart Friday evening, the test pilots of Virgin Galactic's spaceship had accumulated decades of experience and thousands of hours of flight experience.

Co-pilot Michael Alsbury, 39, died in the disaster. His body was discovered among the wreckage of SpaceShipTwo strewn across the Mojave desert in California, according to the Kern County sheriff's office.  The other pilot, Peter Siebold, 43, ejected from SpaceShipTwo and parachuted to the ground, where he was found by Kern County deputies.

Siebold suffered "moderate to severe injuries" and was airlifted to a local hospital, where he is being treated. According to Scaled Composites, which created the rocket plane for Virgin Galactic and employed both test pilots, Siebold is alert and talking with family members and doctors.

The flight was testing a new, plastic-based fuel. Previous SpaceShipTwo flights had relied on a rubber-based fuel, but the company announced the change to plastic in May -- with Virgin Galactic CEO George Whitesides saying the new fuel type would help send the spacecraft higher.

In a press conference after the crash, Scaled President Kevin Mickey said the change was notexpected to cause problems. “We were flying a rocket motor today that had been thoroughly tested on the ground and had gone through a qualification series, so we expected no anomalies with the motor today,” he said.

The cause of the crash remains under investigation by the National Transportation Safety Board. Christopher Hart, acting chairman of the NTSB, said at a news conference late Sunday that the agency was not yet able to pinpoint a cause but that the investigation revealed that Alsbury initiated one step of a two-step process intended for braking the craft, known as "feathering," at the wrong point in the flight. However, the second step that should have been needed to start the braking process occurred without either pilot initiating it, he said.

While the investigation continues, a picture of two deeply experienced engineers has emerged.

Alsbury worked for Scaled Composites for more than a decade and served as a co-pilot on SpaceShipTwo at least seven times between 2010 and 2014, according to the company's flight logs, including last year, when the vehicle first broke the sound barrier.

He was a "home-schooled, home-brewed" pilot who started at Scaled as an engineer and put himself through commercial pilot school, former colleague Brian Binnie, a test pilot who worked at Scaled for over a decade before leaving earlier this year, told the Associated Press.

Alsbury had been a test pilot for 15 years and had more than 1,800 hours of flight experience, according to a biography on the Society of Flight Test Engineers Web site. He also held an aeronautical engineering degree from California Polytechnic State University at San Luis Obispo.

Siebold also held an aeronautical engineering degree from California Polytechnic State University, according to a biography on Scaled's Web site.  He is the director of flight operations at Scaled, where he has been working since 1996.

He was also involved in the creation of an earlier Virgin Galactic prototype, dubbed SpaceShipOne, developing the simulator, navigation system and ground control system. Siebold has 17 years of experience as a test pilot and more than 2,000 hours of flight experience in 35 fixed-wing aircraft, according to the company.

In a statement, Virgin Galactic, which hopes to provide spaceflights to space tourists, said it is not in a position to comment on the crash but expressed condolences to the families and friends of the pilots.

Virgin Galactic backer Richard Branson has previously cited the reentry process as one of the company's trickiest challenges — and suggested that any level of mortality was a potential major problem for its mission.

"Nasa has lost about 3% of everyone who's gone into space, and re-entry has been their biggest problem," he told Britain's Guardian newspaper in an interview published in February. "For a government-owned company, you can just about get away with losing 3% of your clients. For a private company you can't really lose anybody."

Friends and co-workers at Scaled launched a crowd-funding campaign with a goal of $75,000 to support Alsbury's widow, Michelle, and their two children, ages 10 and 7.