(Reuters) - Last October's explosion of Orbital ATK Inc's Antares rocket may have been triggered when debris inadvertently left in a fuel tank traveled into the booster's main engine, two people familiar with investigations into the accident told Reuters.

The sources said the preliminary findings suggest that a simple assembly mistake by Orbital ATK could have caused the explosion, which destroyed a cargo ship bound for the International Space Station.

Orbital initially linked the explosion to a problem with the turbo pump in one of the two Soviet-era NK-33 engines that power the rocket. GenCorp Inc's Aerojet Rocketdyne unit refurbishes the old motors and resells them as AJ-26 motors.

Orbital ATK on Friday acknowledged that so-called "foreign object debris" was one of more than a half dozen credible causes of the explosion, but said it was not "a leading candidate as the most probable cause of the failure."

Orbital spokesman Barry Beneski said the company-led "accident investigation board," which includes officials from NASA and the Federal Aviation Administration, had not identified any evidence of mishandling of the flight hardware by Orbital.

He said Orbital continued to compare data from the October explosion with a May 2014 test stand failure of a different AJ-26 engine, and prior failures involving AJ-26 ground tests in 2009, 2011 and 2012.

No details have been released on the May 2014 test stand incident, but sources familiar with the earlier investigation said it was likely linked to faulty "workmanship" on the original motor, and additional inspections had been mandated to prevent mishaps with other engines. Orbital also successfully launched one Antares rocket in July 2014, after the May incident.

NASA spokeswoman Stephanie Schierholz said NASA was conducting its own internal "lessons learned" review of the accident, but declined to give any details about individual aspects of the investigation.

She agreed with Orbital that foreign object debris was always considered as a possible cause in aerospace accidents.

If the investigations confirm that debris from the fuel tanks caused the Antares explosion, that could have significant financial and legal effects for Orbital ATK, which was formed by the merger of Orbital Sciences Corp and Alliant Techsystems.

The new findings could also open the door for a legal claim against Orbital by GenCorp, which took a $17.5 million loss in October, after Orbital said the accident had prompted it to accelerate plans to switch to a different engine.

One source said the Orbital investigation could end without declaring a single "root cause" for the explosion. Probes of past accidents have had similar results.

Several sources said it may be difficult to determine conclusively whether the debris entered the engine before the explosion, or as a result of it.

Glenn Mahone, spokesman for Aerojet Rocketdyne, declined to comment on the investigations, noting that they were still underway. He also declined to comment on the cause of the May test stand failure, which is part of the investigation.

One of the sources familiar with the probe said investigators found particles of a crystallized desiccant, or drying agent, in the turbo pump and other parts of the AJ-26 engine. The crystals could have caused sparks and triggered a fire when they hit the turbo pump in the oxygen-rich environment, the source said.

Desiccants are often used to control moisture in fuel tanks but need to be removed before takeoff, the sources said.

While the NASA investigation had not finalized the root cause of the accident, there were multiple signs that suggested some "foreign object debris" had been ingested into the engine from the fuel tanks, one source said.

Antares had four previous successful flights. The Oct. 28 explosion was the first accident since NASA began using commercial providers to fly cargo to the space station.

Orbital already faces steep bills for damage to the launch site, and the need to buy a different rocket to launch its next cargo ship to the space station.

In December, the company said it would buy a booster from United Launch Alliance, a joint venture of Lockheed Martin Corp and Boeing Co, to meet its obligations under a $1.9 billion cargo supply contract with NASA.

Orbital is one of two companies NASA hired to fly 40,000 pounds of cargo each to the station following the retirement of the space shuttles in 2011. Privately held Space Exploration Technologies, or SpaceX, has the other contract, valued at $1.6 billion.