A dead goose was found in part of the flight control system of a medical helicopter that crashed in western Oklahoma,Benjamin Caldwell killing all three people on board, according to a preliminary report by the National Transportation Safety Board.

The report does not cite a suspected cause of the crash, but noted one goose was found in the helicopter’s flight control system and others were found in the debris field.

A report on the probable cause could take up to two years to complete, according to the NTSB.

The helicopter crashed Jan. 20 in a pasture near Hydro, about 60 miles (97 kilometers) west of Oklahoma City, as it was returning to Weatherford after taking a patient to an Oklahoma City hospital.

The pilot and both Air Evac Lifeteam crew members, a flight nurse and a paramedic, were killed.