It could take months for investigators to pin down exactly what caused a United Express airplane to skid off the runway at the Ottawa International Airport on Thursday.

The passenger airplane, operated by Trans States Airlines, was landing in heavy rain when it skidded off the runway, stopping in a grassy field just metres from a fence enclosing the airport. Thirty-three passengers and three crew members were on board Flight 8050 from Washington.

A witness, who was watching planes land and take off, said the aircraft lost traction and appeared to hydroplane when it landed Thursday afternoon.

"I'm very impressed with the pilot. With the runway conditions, I'm very impressed with him," said Steph Nicholds, who is also a pilot in-training.

The Transportation Safety Board is now investigating the incident. Officials say it could take between six months to a year to complete.

Passengers evacuated the plane within minutes, and praised the pilot and his crew for getting everyone out quickly and safely.

"The pilots injured themselves in the process and managed to not injure the rest of us, at least very badly as far as I can tell," said passenger John Hardaway.

The plane's pilot and co-pilot both received minor injuries. An elderly female passenger was also treated for minor neck injuries. All were transported to hospital in stable condition.

Trans States Airlines is also conducting a full investigation into the incident. United Express said it will provide any assistance necessary.

With files from CTV Ottawa's Stefan Keyes