Canadian investigators said on Tuesday they sent black boxes for lab analysis from a Delta Air Lines regional jet that flipped upside down upon landing in Toronto a day earlier, as they probe causes of the crash that injured 21 people.

Following initial impact on the runway at Toronto's Pearson Airport, parts of the CRJ900 aircraft separated and a fire ensued, Transportation Safety Board of Canada Senior Investigator Ken Webster said in a video.

The team of over 20 Canadian investigators are leading the probe into the jet operated by Delta's Endeavor Air subsidiary and are getting assistance from the US National Transportation Safety Board and regulators Transport Canada and the US Federal Aviation Administration.

ALSO READ: Delta plane flips upside down on landing in Toronto, injuring 18

Webster echoed other aviation safety officials in saying it was too early to tell what happened to Flight 4819 from Minneapolis-St. Paul International Airport, which had 80 people on board including crew.

Air crashes are usually caused by multiple factors.

The reported weather conditions at the time of the crash indicated a "gusting crosswind and blowing snow," flight tracking website FlightRadar24 said.

In a video shared widely on social media showing the plane's descent, the landing appeared flat and did not show the regular "flare" maneuver, where pilots pull the nose up to increase pitch just prior to touchdown to slow speed, experts said.

"The question is why was it so firm," US aviation safety expert John Cox said of the landing, but stressed that investigators are still gathering data and evidence.

"The analytics start later," he said.

The 16-year-old CRJ900, made by Canada's Bombardier and powered by GE Aerospace engines, can seat up to 90 people. At least one of the two wings was no longer attached to the plane, video showed after the accident.

No fatalities

Delta said earlier on Tuesday that 19 of 21 passengers transported to local hospitals after the plane crash had been released.

Greater Toronto Airports CEO Deborah Flint told reporters the two remaining hospitalized passengers did not face life-threatening injuries from the crash, which had no fatalities.

"It's really, really incredible when you see that aircraft - it just makes you really thankful for all the safety checks," Flint said, praising the flight crew and first responders.

Passenger Peter Koukov, 28, a professional skier and videographer from Denver, told Reuters he did not know anything was wrong until the jet hit the ground.

"It then kind of like bounced and almost felt like we were, like we were lifting off again and we turned on our side, slid on our side for a little bit and then ended up upside down,” said Koukov, who was flying through Minneapolis to Toronto to help film a ski video.

Koukov said he and a female passenger besides him remained calm and carefully lowered themselves down.

"We didn't talk the ... entire flight, and we just both just, like, hugged, like, a long, like, 10-second hug,” he recalled. "And then we're like, okay, like, what's next? Like, we need to get off this plane."

The Delta plane touched down in Toronto at 2:13 pm (1913 GMT) on Monday after an 86-minute flight and came to rest near the intersection of runway 23 and runway 15, FlightRadar24 data showed.

The wreckage of the plane is expected to remain on the airport grounds for potentially another 48 hours and two of Toronto Pearson's runways will remain closed until it is removed, Flint said.

The reductions in the number of takeoffs and landings, combined with delays and cancellations due to a weekend snowstorm that dumped more than 22 cm (8.6 inches) of snow at the airport have weighed on efforts by Air Canada to restore service after canceling about 1,290 flights over the past six days.

"We anticipate it may take several more days, depending on the weather, to return to fully normal operations," Air Canada Chief Operating Officer Craig Landry said in a statement.

Air Canada, the country's largest carrier, said on an average day that almost half of its flights and customers pass through Toronto Pearson airport.