AMERICA's National Air Cargo declined to comment on the cause of the 2013 Bagram Air Base 747F crash because of hearings before the National Transportation Safety Board that open on July 14.

Questions arose after the US Federal Aviation Administration (FAA) proposed a US$77,000 fine for having an insecure load of armoured vehicles aboard, which shifted on takeoff causing the crash that killed seven aircrew.

Reached by the Wall Street Journal, company spokesman Mark Dombroff said the National Air Cargo could not comment on the cause of the crash because it was under NTSB investigation.

"Certainly our goal is to discuss the penalty and the investigation and to inform them of certain facts the FAA may not be aware of," said Mr Bombproof.

"We are still waiting to get information from them about their basis for the findings," he said.

Mr Dombroff said the flight had originated at Camp Bastion, a separate military base in southeast Afghanistan and that the vehicles had been loaded onto the 747 there.

"The takeoff from Bastion was completely uneventful, routine, as was the landing at Bagram," Mr Dombroff said.