Pilot Michael Alsbury was killed in the Virgin SpaceShipTwo crash in Mojave Desert over the California on October 31. (Photo by David McNew/Getty Images)

They were close friends whose children often played together on weekends and who had spent thousands of hours screaming through the sky.

Peter Siebold was a naturally gifted flier whose father, who owned a plane, would perch him on a stack of pillows in the cockpit when he was five so he could experience the thrill of flight as if he were the pilot.

Michael Alsbury, like his friend, also was largely a self-taught flying obsessive, wanting his pilot’s license far more than his driver’s permit.

They were engineers with degrees from the same university and test pilots working for the same company. And on Friday they were pushing the frontier of commercial space travel together when their Virgin Galactic spacecraft crashed in the Mojave Desert, killing Alsbury, 39, and sending Siebold, 43, to the hospital with an injury to his shoulder.

The crash is being investigated by the National Transportation Safety Board, which said it has an unusually rich trove of data and resources to mine. At a news conference Sunday night, the NTSB’s chief investigator raised the possibility of pilot error, saying the co-pilot unlocked a mechanism that enabled the back half of the spaceship’s wing — the part known as the feather — to bend along its hinge at the wrong time and speed.

While stressing that investigators have not determined the cause of the crash of the Virgin spacecraft known as SpaceShip­Two, Christopher A. Hart, the acting director of the NTSB, said there was no sign of an engine failure, fire or explosion. At another news conference late Monday, Hart said debris had been found as far away at 35 miles from the crash site.

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Employees at Scaled Composites, the firm that designed the space plane, on Friday were able to watch video camera feeds from inside the cockpit and outside the spaceship during its flight.

“There are dozens of reasons why mistakes like this one could be made,” said another Scaled test pilot.

That pilot went on to explain that there was a rule that anyone flying the spaceship could not re-configure the vehicle without the verbal acknowledgment of both pilot and co-pilot. It is unclear whether that protocol was followed. Normally, the co-pilot would announce when Mach 1.4 had been reached — the proper speed to unlock the feather. The pilot would acknowledge and command the co-pilot to unlock the feather. Once the feather was unlocked, the co-pilot would announce the maneuver had been completed.

A number of sources, who spoke on the condition of anonymity because the company has forbidden interviews with the media, described seeing Alsbury unlock the feather and then appear to realize there was an error, moving quickly as if he was trying to shut off the motor, but it was too late.

These sources said that within the company, there is a growing recognition that Alsbury, the co-pilot, unlocked the feather early, although it is not clear why. Colleagues say Alsbury was one of the sharpest test pilots on the team, with more than 1,600 hours of flight experience in more than nine different aircraft.

The first 15 seconds of the burn are incredibly chaotic for the pilot and co-pilot. Lighting the rocket motor is akin to opening the gate at a rodeo — the bull starts bucking and the pilot is just along for the ride, tossed back into the seat with three times the force of gravity and pushed down with even more force.

During this period, the pilot and co-pilot go through their flight-test cards, performing a number of “housekeeping” duties meant to prepare the ship for the remainder of the flight.

The feather is critical for the plane’s descent back to Earth, when the engine automatically shuts off and the plane glides down. The feather allows it to descend without building up too much speed or too much heat, and it also ensures the spaceship stays right-side up.

But unlock the feather too soon — as Hart says Alsbury probably did at Mach 1.0 — and the results are catastrophic.

In an interview Monday, Hart said the agency’s probe would be exhaustive and look at the pilots’ training, fatigue levels and experience. He said that because it was a test flight, the NTSB had lots of witnesses, camera footage and data that would help it “find out what went wrong and how to prevent it from happening again.”

“There’s much more that we don’t know,” he said Sunday night at a news conference when asked if there was pilot error. “I’m not saying this was the cause,” he said repeatedly.

As long as humans have pushed the limits of flight, there have been accidents, many fatal. And the progress of the past century has been advanced by failure as much as success, aerospace officials said.

Most test pilots are not the reckless adventurers they are made out to be, said Ron Kaplan, the enshrinement director for the National Aviation Hall of Fame.

“They are actually the polar opposite,” he said. “They are very methodical and very well trained, and the risks that are taken are clearly calculated risks. But that said, the price of progress and the cost of being on the cutting edge, unfortunately, is sometimes counted in lives.”

Siebold greatly admired aeronautical pioneers — from the Wright Brothers and Glenn Curtiss to Chuck Yeager. He was so eager to work for Scaled Composites that he left California Polytechnic State University without his degree. He wanted to work for another of his heroes, Scaled Composites’ founder Burt Rutan, an engineer heralded for designing strong but lightweight aircraft. And soon Siebold landed the coveted job of test pilot.

Alsbury also wanted to work at Scaled Composites to follow the footsteps of Rutan. He began as a flight-test engineer and worked his way to being a test pilot. He became a certified flight instructor and learned as much as he could to prove his flying skills. Just before Rutan retired, his last program was a flying car, and Alsbury was picked as the test pilot for the vehicle.

Fred Barbash and Drew Harwell contributed