The pilot who died in a northern Minnesota plane crash earlier this month may have been suffering a lack of oxygen when his plane went down in Leech Lake near Walker.

Bad weather may have also been a factor in the Sept. 1 crash that killed Thomas Borum, according to a preliminary report issued over the weekend by the National Transportation Safety Board.

Borum, 62, was a veterinarian who was relocating from Mississippi to Minnesota to work at the Bemidji Veterinary Hospital's satellite Lake of the Woods Veterinary Clinic in Roseau, Minn. On the day of the crash, Borum left Natchez, Miss., around 9:12 a.m. and stopped to refuel in Kirksville, Mo. He departed there around 2:36 p.m. with a final destination of Bemidji, the report said.

Air traffic controllers became concerned with Borum's altitude, course deviations and slow communication responses. Believing he was suffering from hypoxia, a condition that results when the brain does not get enough oxygen, controllers directed him to the St. Cloud Regional Airport, the report said.

Borum refused medical treatment in St. Cloud and, after about an hour on the ground, took off for Bemidji, the report said.

Strong storms were detected over Borum's intended route and radar indicated that he made several course corrections as he approached the airport, including two 360-degree turns. He turned and flew away from the airport at one point, then turned again and flew toward it. His altitude fluctuated between 2,000 and 7,000 feet before the plane made a "continual descent toward the ground," the report said.

A search for Borum began around 9:15 p.m. when he failed to arrive in Bemidji.

The wreckage was found the next morning in Leech Lake, about 31 miles southeast of Bemidji, in about 12 feet of water. The fuselage and tail section were intact; the wings had separated from the airplane but were still connected by cables, the report said.

A final crash report is not expected for 12 to 18 months, and information in the preliminary report is subject to change, the NTSB said.

Tim Harlow • 612-673-7768