Kansas City, Mo. ? The pilot of a medical helicopter that crashed in a Missouri field had reported that it was low on fuel, federal transportation officials say, but it could be as much as a year before they know what caused the crash that killed all four people onboard.

The Eurocopter AS-350, owned by Colorado-based Air Methods Corp. and flown by its subsidiary, LifeNet, crashed just before 7 p.m. Friday two miles from Mosby, where it had planned to land at the Midwest National Air Center for refueling. The helicopter’s ultimate destination was a suburban Kansas City hospital, about 10 miles from Mosby.

The pilot, two medical crew members and a 58-year-old female patient were killed.

The helicopter had flown from St. Joseph, where it is sometimes based at Heartland Regional Medical Center, to Bethany, near the Iowa border, to pick up the patient before heading to Liberty to drop her off.

National Transportation Safety Board spokesman Peter Knudson said there was no distress call from the helicopter and no witnesses to the accident.

Lynn Lunsford, a spokesman for the Federal Aviation Administration, said it was too early to determine whether fuel had anything to do with the crash.

Air Methods Corp. on Saturday identified crew members as Randy Bever, 47, a flight nurse from Savannah, Mo.; Chris Frakes, 36, a flight paramedic from Savannah; and James Freudenbert, 34, a pilot from Rapid City, S.D.