A WestJet Encore flight from Vancouver to Nanaimo had to be evacuated on the tarmac Tuesday after the cabin started filling with smoke moments before landing.

Passengers on Flight 3161 said the smoke began building while the smaller Bombardier Q400 aircraft was approaching Nanaimo Airport.

"It was getting worse and worse," said Robin Thacker. "They told us to breathe through our clothing."

Thacker captured video of the hazy cabin as crew members searched for the source of the smoke and instructed passengers to stay seated.

Once the plane landed, an announcement came over the speakers instructing everyone to, "Evacuate. Evacuate. Evacuate."

Another passenger, Marcie Pimlott, said there were a few tense moments before rattled travellers were able to get out, but praised the crew for remaining calm.

"People started to panic a little bit," she said. "It was very intense, pretty scary."

The Transportation Safety Board has since deployed a team of investigators to determine the source of the smoke. The probe is still in its early stages, but the TSB’s regional manager Yanick Sarazin said they are looking into whether the smoke might have come from plane's left engine.

"It's inconclusive for now, but that is the belief for now so we're concentrating on that," Sarazin said.

WestJet confirmed an emergency was declared onboard so the plane could get priority landing at the airport. Once the doors opened, two flight attendants and two pilots worked to get the 56 passengers onto the runway and away from the aircraft.

Warren Le Grice, a former air traffic control and volunteer pilot for Angel Flight, said it appears the crew reacted to the alarming situation appropriately.

"They responded by the book," he said. "Sounds like they did everything right." 

Some passengers questioned why oxygen masks weren't deployed during the incident, but experts say they don’t form a tight seal from cabin air, and typically aren't used to protect passengers from smoke. Instead, oxygen masks automatically deploy during cabin depressurization, while the aircraft descends to a more comfortable altitude. 

The Q400 has a maximum altitude of 25,000 feet, which according to Transport Canada regulations means it isn’t required to have oxygen masks for every passenger. WestJet said the deck on Flight 3161 is equipped with oxygen masks and that portable oxygen is available for passengers. 

The company wouldn’t say if there was smoke in the cockpit during Tuesday's incident, or if the pilots donned their oxygen masks, which they are quickly trained to do in an emergency. It could not comment on what happened but said "a preliminary maintenance inspection is being performed."

"We appreciate and recognize that this would be an unsettling experience for all involved and we apologize to our guests for this," WestJet said in an email statement. 

Three subsequent flights between Vancouver, Nanaimo and Comox were cancelled because of the incident, according to the airline.

With files from CTV Vancouver's Penny Daflos and David Molko