Investigators know of four vehicles that were hit by pieces of metal about the size of a cellphone around 2:30 p.m. ET, said Peel regional police Const. George Tudos.
Witnesses reported seeing a plane with smoke coming from one of its engines shortly before police got calls about the falling debris, Tudos said.
"As it (the plane) was travelling away from Pearson we had other complaints stating that debris, consisting of metal objects, was falling from the sky."
Tudos said there were no injuries and stressed that police can't positively say the debris came from the Air Canada plane.
"We believe it is, but it's not up to us — we're not the investigating body," he said.
No one on the ground was injured by the falling metal, Tudos said.
An Air Canada spokesman said a Boeing 777 headed for Japan was forced to return to Pearson shortly after takeoff when one of the two engines failed.
The plane landed normally on one engine, Peter Fitzpatrick said, adding there were no injuries among the 318 passengers and 16 crew.
A businessman who was on board the plane tweeted that passengers were told it was an engine overheating.
"Seems my plane fell apart! Luckily we managed to land it," Jason Flick tweeted, adding that the plane spent 20 minutes dropping fuel and landed heavy.
"What sucks is hundreds of people tucked into a small area and no air-conditioning," Flick said in his final tweet.
The cause of the engine shutdown on flight AC001 to Narita airport in Tokyo wasn't immediately known, Fitzpatrick said.
The Transportation Safety Board of Canada said an investigator has been sent to Pearson airport to gather information related to the emergency landing.
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