Three people were killed and 19 injured Thursday in a fiery, mid-day chain reaction crash on a busy street in Calgary that left two transit buses on fire, authorities said. Police Staff Sgt. Andy Woodward says the tragedy appears to have begun after a transit bus slammed into a car that then hit other vehicles. He says the bus then bounced from that crash, through the intersection, and hit a transit bus parked at a bus stop, setting off a fire and sending the second bus across the road to collide with three more vehicles, the AP reports. He says the second bus was powered by natural gas, which is believed to have caused the fire.

Video and photos circulating online showed the second bus with its roof completely on fire as thick smoke billowed skyward. The front end of the bus that set off the chain reaction crash appeared crumpled and smashed, its steering wheel hanging out below a tuft of flames. “I’ve been a police officer for 36 years and I think this is probably the biggest scene I’ve worked on in relation to the collision we’ve got here,” Woodward says. One of the 19 people injured in the crash suffered life-threatening injuries, authorities say. A police officer was also injured as she ran toward the burning buses.

Calgary Fire Chief Steve Dongworth says 12 units responded to the scene, the CBC reports. “It was fairly involved. It wasn’t a catastrophic fire, but it was a serious fire inside,” Dongworth says. “There was also a small fire in the other bus.” Dongworth says the driver of the second bus was trapped and had to be extricated by firefighters. Amalgamated Transit Union Local 583 president Mike Mahar tells the Calgary Herald that the the scene was “shocking” and he’s never seen a bus burn like that before. He says the union hasn’t previously had safety concerns about the city’s fleet of more than 200 natural gas-powered buses, but “we may now.” Authorities say there will be a full investigation.