A single-engine airplane crashed on a hill about a mile from Danbury Airport Sunday morning, and sent three unidentified men to Danbury Hospital with varying injuries.

According to several reports, one of the men was in life-threatening condition following the crash and another was in severe condition. A third man was treated for non-life-threatening injuries, reports said.

As of Press time Wednesday, the one man was still in critical condition.

The cause of the accident remains undetermined, and the names of the injured have not been released.

National Transportation Safety Board (NTSB) detectives took over the crash investigation from Danbury police and removed the Cessna Skyhawk Monday. NTSB officials have yet to release any information about what led to the accident.

According to a source who lives in New Canaan and owns a hanger at Danbury Airport, weight on board the aircraft might have been a factor in the crash.

“What I’ve heard is that a student pilot went up with an instructor and a third person and that they were probably overweight, and they couldn’t get enough lift to clear the trees,” the source told The Press Monday afternoon.

The source said that Cessna 172 Skyhawk is regularly used by students as a “trainer plane.”

Stephen Williams, the deputy fire chief of the Danbury Fire Department, told The Danbury News Times that the fuel tank was full when the four-passenger plane went down.

According to that report, the plane had been rented from Arrow Aviation — a flight school on Miry Brook Road — for a local flight and took off from a runway behind the FedEx building on Backus Avenue.

Dick Aarons, Ridgefield’s emergency management director, told The Press Tuesday that the official NTSB report was “months away” — 12 to 18 months — from being completed.