The alert sys­tem on a Delta Air Lines jet that flipped up­side down and burst in­to flames as it tried to land in Toron­to last month in­di­cat­ed a high rate of de­scent less than three sec­onds be­fore touch­down, a pre­lim­i­nary re­port said Thurs­day.

The Trans­porta­tion Safe­ty Board of Cana­da, which is­sued the re­port, con­tin­ues to in­ves­ti­gate the Feb. 17 crash-land­ing in which 21 peo­ple were hos­pi­tal­ized.

All 76 pas­sen­gers and four crew mem­bers sur­vived when the Delta plane ar­riv­ing from Min­neapo­lis burst in­to flames af­ter flip­ping over and skid­ding on the tar­mac.

The TSB of Cana­da re­port says that when the plane’s ground prox­im­i­ty warn­ing sys­tem sound­ed 2.6 sec­onds be­fore touch­down, the air­speed was 136 knots, or ap­prox­i­mate­ly 250 kph (155 mph). It says the plane’s land­ing gear fold­ed in­to the re­tract­ed po­si­tion at touch­down and the wing de­tached from the fuse­lage, re­leas­ing a cloud of jet fu­el, which caught fire as the plane slid along the run­way.

The fuse­lage rolled up­side down and a large por­tion of the tail came off in the process, the re­port says.

“Ac­ci­dents and in­ci­dents rarely stem from a sin­gle cause,” TSB chair Yoan Mari­er said in a video state­ment Thurs­day. “They’re of­ten the re­sult of mul­ti­ple com­plex, in­ter­con­nect­ed fac­tors, many ex­tend­ing be­yond the air­craft and its op­er­a­tion to wider sys­temic is­sues.”

The crew and pas­sen­gers start­ed evac­u­at­ing once the plane came to a stop, the re­port says, adding that some of the pas­sen­gers were in­jured when they un­buck­led their seat­belts and fell to the ceil­ing.

The TSB says it’s not aware of any is­sues with the seat­belts or seats dur­ing the in­ci­dent.

The cock­pit door was jammed shut, forc­ing pi­lots to es­cape through the emer­gency hatch on the ceil­ing of the cock­pit af­ter every­one else was out, the re­port says.

Emer­gency re­sponse per­son­nel then went in­to the fuse­lage, and there was an ex­plo­sion out­side the plane near the left wing root short­ly af­ter­ward, the TSB says. The cause of the ex­plo­sion has not yet been de­ter­mined.

So far, the in­ves­ti­ga­tion has found no pre-ex­ist­ing prob­lems with the flight con­trols, though some com­po­nents were dam­aged in the crash, the board said.

The safe­ty board says its on­go­ing in­ves­ti­ga­tion is fo­cus­ing on sev­er­al key ar­eas, in­clud­ing met­al­lur­gi­cal ex­am­i­na­tion of the wing struc­ture, land­ing tech­niques, pi­lot train­ing and the pas­sen­ger evac­u­a­tion process.

All of those who were hos­pi­tal­ized were re­leased with­in days of the crash.

At least two law­suits have been filed in the Unit­ed States, and a law firm in Cana­da has said that it’s been re­tained by sev­er­al pas­sen­gers.

Delta de­clined to com­ment on the pre­lim­i­nary re­port.

“We re­main ful­ly en­gaged as par­tic­i­pants in the in­ves­ti­ga­tion led by the Trans­porta­tion Safe­ty Board of Cana­da. Out of re­spect for the in­tegri­ty of this work that will con­tin­ue through their fi­nal re­port, En­deav­or Air and Delta will re­frain from com­ment,” the air­line said.