Delta Air Lines confirmed Tuesday that 19 of the 21 individuals who were transported to Toronto area hospitals following Monday’s plane crash have been discharged. Investigators continue to look into why the regional jet rolled over on landing.

Canada’s Transportation Safety Board is investigating the accident, which occurred on a CRJ900 plane operated by Delta’s Endeavor Air subsidiary.

The plane, traveling from Minneapolis-St. Paul International Airport as Flight DL4819, had landed at Toronto Pearson Airport and for unknown reasons landed upside down.

The cause of the accident is not yet known since airplane crashes are normally caused by a combination of reasons.

The aircraft is a 16-year-old Bombardier CRJ900 with GE Aerospace engines and a capacity of carrying 90 passengers. The left wing of the aircraft was observed to have been detached after the crash in video footage around the scene.

Toronto Pearson Airport was dealing with frigid temperatures and blustery winds on Monday, as airlines struggled to recover from weekend snowstorm delays that deposited over 22 cm (8.6 inches) of snow.

The Delta aircraft landed at 2:13 p.m. local time (1913 GMT) after a 86-minute flight, according to FlightRadar24. It came down near where runway 23 crosses runway 15. The weather at the time, which was gusty crosswinds and blowing snow, may also have played a role, FlightRadar24 said.