Calgary Transit says it will review its compressed natural gas bus operations after a crash and fire killed three passengers Thursday, CBC News and the Calgary Herald reported. Sixteen people were injured, CBC News and the Herald reported.

The crash happened just after 1 p.m. on Saddletowne Circle N.E. in the city's northeast, CBC News reported. Police said a transit bus struck a truck, then hit the back of a sedan and pushed it through an intersection, CBC News and The Canadian Press reported. The bus then struck the back of a second transit bus, CBC News reported.

The natural gas tanks on the second bus ignited, police said, CBC News and the Herald reported. "Within seconds, the fire intensified resulting in an explosion and significant fire inside the bus," police said in a news release, according to the Herald and The Canadian Press. Three passengers on the second bus were found dead, police said.

The driver of the first bus was trapped and was taken to hospital with life-altering injuries, CBC News reported. Ten people went to hospital by ambulance, one in life-threatening condition, CBC News reported. A police officer suffered minor injuries, CBC News reported.

Calgary Transit said its 347 natural gas buses get regular inspections and safety checks, CBC News and the Herald reported. "We will be working closely with the investigators to understand the circumstances surrounding the incident and will conduct a review of our CNG operations to ensure any additional safety measures required are put in place," the agency said, according to the Herald.

It is too early to say whether impairment, speed or distraction played a role, police said, according to CBC News and The Canadian Press. Bus maker Nova Bus said it was cooperating and that it would be premature to guess at a cause, CBC News and the Herald reported. "We don't believe there should be a stage where the system could fail to this level," said Mike Mahar, president of the transit workers union, according to the Herald.

What we know: Three people died, 16 were hurt, and Calgary Transit is reviewing its natural gas buses.
What we don't: What caused the crash, and why the fire spread so fast.