ALAMEDA COUNTY (BCN) — A pilot who died when his small plane crashed in Alameda County last month was descending into Oakland International Airport but was having difficulty following the directions of air traffic controllers, according to a preliminary report released by the National Transportation Safety Board.

What caused the crash that killed 61-year-old John Sacco of Rocklin in a rural area of Castro Valley on Dec. 22 remains under investigation, but at one point Sacco told air traffic controllers by radio that he was “too high” when he was in fact flying at a lower altitude than recommended, according to the NTSB.

Just before he crashed, Sacco told controllers over the radio that he was “losing it,” the report states.

The airport lost contact with Sacco at about 11:30 a.m. and the plane went down in a steep wooded area off Palomares Road. The wreckage path extended 300 feet down a hillside and the cabin and cockpit were consumed by fire.

It took about four hours for firefighters and sheriff’s deputies searching the area to find the wreckage because they were unable to search by helicopter due to fog.

Sacco was coming from Lincoln in Placer County in his Piper PA-32 single-engine plane. He had been cleared to land at the Oakland airport and was instructed by air traffic controllers to turn 260 degrees, descend and maintain an altitude of 3,400 feet, according to the NTSB.

The plane turned, but was off course. As Sacco worked with the air traffic controller to try to get the plane back on the proper course, he said was “too high,” but was actually flying at an altitude of 2,600 feet — far lower than the 3,400 feet minimum altitude for that segment of the approach, according to the NTSB.

The controller issued a low altitude alert and cancelled Sacco’s approach clearance, instructing him to turn right and climb to an altitude of 4,000 feet. But he didn’t climb and the controller instructed him to turn again. Eventually he climbed to 3,600 feet but began descending again, according to the NTSB.

Just before the crash, air traffic controllers received a partially blocked transmission of Sacco saying he was “losing it.” Radar contact with Sacco was lost about a minute later.

Sacco was appointed to the state Occupational Safety and Health Standards Board by Gov. Jerry Brown in August. He was an industrial hygienist and safety engineer at Earthshine Consulting since 1998, according to Brown’s office.

The NTSB is continuing its investigation into the crash and is expected to issue a final report in the coming weeks or months.