A preliminary report from the National Transportation Safety Board (NTSB) indicates that weather may have played a role in a northern Minnesota plane crash that killed a well-known Twin Cities doctor.

Snow was falling and the temperature was just above freezing when Thomas Stillwell took off from Moose Lake Carlton County Airport on the afternoon of May 8. He was bound for Crystal but never made it.

Stillwell, 65, of Plymouth, crashed into the Moose Horn River northwest of the airport. His single-engine plane was found the next morning along the west bank of the river in 2 to 4 feet of water. The engine and propeller were embedded in the mud and the tail was sticking up in the air, according to the NTSB report.

The report did not definitively say weather caused the crash, but it focused on the deteriorating conditions. Investigators offered this timeline:

Stillwell obtained a weather briefing at 3:48 p.m. He left a hospital in Moose Lake at 4 p.m. and drove to the airport seven minutes away. Stillwell was given clearance to take off, but that clearance was scheduled to expire at 4:35 p.m., the report from lead investigator Jennifer Rodi said.

The report did not say what time Stillwell took off from the airport, but “when the pilot did not check in with air traffic control, a search for the airplane and pilot was initiated,” it read.

A nearby weather observation station reported that snow had started falling, winds were blowing at 12 mph with gusts to 18 mph and that visibility was 2½ miles at 4:15 p.m. Twenty minutes later, when Stillwell’s clearance to fly was to expire, conditions had worsened. Snow was still falling and visibility had dropped to 1¼ miles. By 4:55 p.m., visibility had fallen to 1 mile and winds continued to blow at between 11 and 17 mph, the report said.

Searchers found Stillwell’s aircraft, a 32-year-old Mooney M20J, in the river the next morning.

Stillwell was a urologist associated with North Memorial and Fairview health systems.

Known as the “flying doctor,” Stillwell spent up to 12 days each month flying to outreach clinics in rural communities to treat patients, including the Minnesota cities of Mora, Onamia, Moose Lake and Sandstone, and Grantsburg, Wis., according to his online obituary.

He was a Navy veteran who served as a surgical field medic in the Gulf War.