The bus, with more than 30 people on board, veered across the road, struck a van and crashed through a guard-rail before falling about 15 metres onto an apartment block.

Fifteen of the passengers were critically injured with another 10 seriously injured.

The driver was thrown from the bus and killed.

The other man killed had been shot in the head, according to police. "Possibly this is our suspect, possibly it's another victim shot by the suspect," said police officer Christie-Lynne Bonner.

"The information we have at this time is that one of the passengers on the bus shot the driver of the bus," she said.

Bus flew through air

The bus, an extra-long articulated vehicle, was heading towards the centre of the city when it tumbled from Aurora Bridge.

Firefighters search the wreckage of the bus

Dave Chacone, who was standing underneath the bridge, said the bus flew right overhead.

"We were outside talking, heard a noise, looked up and saw this bus coming," he said. "I looked up and saw some big chunks (of concrete railing) coming right at me," he said.

Fire Department spokeswoman Sheila Strehle said the 30 passengers were taken to six different hospitals in the area.