It's too soon to know how and why an Air Canada plane arriving from Halifax slid off the runway at Toronto's Pearson International Airport early Saturday — but investigators aren't ruling out that weather may have been a factor, the Transportation Safety Board says.

At a news conference Saturday, TSB regional manager of air investigations Ewan Tasker said two investigators were deployed Saturday morning to gather information about the incident, which took place shortly after midnight in heavy fog and rain.

Initial findings show that during its landing sequence, Flight 623 travelled through the grass on the western side of the runway, eventually coming to a stop on the centre line, said Tasker. The flight carried 118 people, though it's unknown if that number includes crew members. No injuries were reported.

"The runway at the time was reported to be damp or wet; there was thundershowers in the area and rain," Tasker said. Whether the wetness was a factor, he said, it's too early to tell.

Early information shows five lights on the western side of the runway were damaged, along with one of the plane's tires, Tasker said. There was minor damage to the aircraft, which will be fully examined. Witnesses reported that the plane's side was covered in mud.

As part of the investigation, the plane's cockpit data recorder and flight data recorder will be sent to a TSB lab in Ottawa for examination. The TSB will also be working to collect thorough weather information with the help of Environment Canada.

Tire 'completely gone,' passenger says

The next steps will include interviewing flight crew, cabin crew, air traffic control and witnesses, as well as gathering information on the plane's approach, crew training and what navigation aids were used.

The Transportation Safety Board is investigating after an Air Canada plane arriving from Halifax with 112 passengers on board slid off the runway early Saturday at Toronto's Pearson International in heavy fog and rain. 0:39

Passenger Paul Varian told CBC News the incident appeared to have damaged the undercarriage of the plane.

"The whole of the undercarriage on my side, on the left-hand side, when we took a look while we were deplaning, was destroyed. The tires were completely gone off the wheel," he said.

Varian said passengers were calm throughout the incident and the flight crew was helpful as passengers got off the plane.

"This is Canada. Everyone was so polite and nice and accommodating."

'Our focus was our passengers'

Isabelle Arthur, media relations manager for Air Canada, said the aircraft's landing gear left the runway for a short distance before it returned to the runway, and passengers were taken in buses to a nearby terminal.

She said the airline is relieved that nobody was hurt.

A passenger who took this photo said the 'clouds were quite low' as the plane landed in Toronto after a flight from Halifax. (Paul Varian)

"Our focus was our passengers that were quickly bused to the airport and met by and tended to by a team of Air Canada employees and managers," Arthur said, adding passengers received hotel accommodations and transportation, if they wished to head home.

"We apologize for the inconvenience and will be in touch with our customers to follow up on the event and ensure they are all well taken care of," Arthur said.

Luggage would be delivered to passengers once officials finish reviewing the incident, said Arthur. According to witnesses, passengers disembarked while on the runway.

Mississauga Fire and Emergency Services, along with Peel Regional Paramedic Services, were also called to the scene.

Mud is clearly visible on the side of Air Canada Flight 623. (John Hanley/CBC)

Const. Harinder Sohi, spokesman for Peel police, said the pilot was able to get the plane back on the runway.

Passengers from Air Canada 623 disembark from the plane on the runway. (John Hanley/CBC)

Passenger Varian, who didn't know the plane had left the runway until he was in the terminal, said the flight was delayed leaving Halifax. He said the clouds were quite low during landing in Toronto and the landing gear "blew."

"We landed heavily, nothing that I would say was traumatically heavily. We just landed quite heavily on the tarmac and the plane lurched a bit to the left. Then the pilot applied heavy brakes. That threw everyone forward a bit," he said.

'Black residue on my window'

The plane came to an abrupt halt on the runway.

"We stopped there. And it was pretty clear to me that something was wrong because I looked out my window and there was a lot of black residue on my window and the windows beside me."

He said that after about an hour and a half, the passengers disembarked and were taken in buses back to the terminal.

<a href="https://twitter.com/hashtag/AC623?src=hash">#AC623</a> slid off the runway after landing, went wheelin', now back on the runway. Passengers deplaning on the runway. <a href="https://twitter.com/hashtag/YYZ?src=hash">#YYZ</a> <a href="https://t.co/CKxL2pNVpj">pic.twitter.com/CKxL2pNVpj</a> —@CodyVella

Jock Williams is an aviation expert who served in the Canadian air force for more than 30 years and was once an instructor for Transport Canada.

Jock Williams is an aviation expert who served in the Canadian air force for over 30 years and was once an instructor for Transport Canada. (CBC)

He's not an investigator but says he has seen wet runways cause planes to go off the runway before due to a phenomenon called hydroplaning — when a layer of wetness prevents tires from making direct contact with the ground's surface, causing them instead to skim the surface.

"They're at the mercy of the laws of physics and the runway and mother nature with the wind. And if all those combine in their favour — they go in a straight line, come to a stop, taxi back in and nobody knows," Williams said.

"A curling stone can go sideways. Well a plane can go sideways, too."