A joint U.S.-Afghan investigation into the crash of a civilian cargo jet in Afghanistan that killed seven Americans has ruled out insurgent involvement, but the exact cause could take up to a year to determine, an Afghan official said Monday.

Investigators from U.S. National Transportation Safety Board and the Afghan Civil Aviation Authority spent 12 days in Afghanistan studying the flight recorder from the Dubai-bound Boeing 747-400 that went down soon after takeoff April 29 at Bagram Air Field, north of Kabul.

The Taliban quickly claimed responsibility, saying it had shot down the plane, but NATO has consistently rejected that, saying there was no insurgent activity in the area.

Nangyalai Qalatwal, spokesman for the Afghan aviation body, said only a few words of the pilot's voice could be understood on the flight recording. Even so, the crash was ruled an accident.

"What is clear for us after this investigation is that the claim of responsibility for the Taliban is false," Qalatwal said Monday. "There was no sound of any rocket attack or any insurgent involvement in that crash. It was only an accident."

He noted that the plane was inspected only two hours before its takeoff and was found to have no technical problems. The American investigators took the flight recorder with them when they returned home, and Qalatwal said it could take up to a year to determine the exact cause.

The plane — owned by National Airlines, an Orlando, Florida-based subsidiary of National Air Cargo — was carrying vehicles and other cargo.

Six of the dead Americans were from Michigan and the seventh was from Kentucky, said Shirley Kaufman, a National Air Cargo vice president.