A woman is fighting for her life after being struck by one of two wheels that flew off a dump truck in Mississauga this morning.

The woman, said to be in her 40s, was walking on the sidewalk of Meadowvale Boulevard, just north of Highway 401, around 8 a.m. when she was struck by the wheel — which likely weighed nearly 200 pounds — according to police.

Diane Tsialtas, of Brampton, was identified by friends as the victim of the accident. Tsialtas, an avid runner, is well-known among that community.

She was airlifted to Sunnybrook Hospital with life-threatening injuries and remains in critical condition.

Peel Regional Police still haven't determined how the tires came loose from the 14-wheel truck.

John Furlano, a truck technician who does safety checks on commercial vehicles including dump trucks, said he believes the nuts that hold the tire in place were loose after looking at pictures of the truck involved.

"That would cause the rim itself to rock back and forth," Furlano said, adding that's something the driver should have detected during his daily check over the vehicle.

"He should have heard something coming from that wheel and maybe stopped and circled the truck."

Police are questioning the driver but haven't laid any charges at this time. 

"It looks like due to the negligent condition of the dump truck the tire flew off and struck a pedestrian who was walking on the sidewalk," said Const. Rachel Gibbs of Peel Regional Police.

After hitting the woman, one wheel bounced about 200 metres into the parking lot of an office building. It's not clear how fast the truck was moving at the time the wheels came loose.