Why did the Delta plane crash and flip in Toronto? Here's what we know about the cause.

A Delta Air Lines jet crashed Monday during a fiery landing at Toronto Pearson International Airport, skidding down the tarmac before coming to a stop while flipped upside down. 

The dramatic scene, captured on video by air traffic controllers and, later, the passengers on board, left 21 people injured — two critically. What caused it remains under investigation by the Canadian Transportation Safety Board and multiple transit agencies in the United States. Here's what we know so far.