CBC News has confirmed the driver of the bus involved in Friday's deadly crash in Ottawa was hired within the last year and was involved in another collision last month.

Sources, speaking on condition on anonymity, have identified Aissatou Diallo as the driver of the double-decker bus that slammed into an overhang at Westboro station, killing three people and injuring nearly two dozen.

CBC News has learned the earlier collision, between a bus Diallo was driving and a hybrid bus, happened at St. Laurent station, just east of the city's downtown.

OC Transpo has so far offered no comment on Diallo's level of experience or her driving record.

The city's general manager of transportation, John Manconi, would not comment Monday on the earlier crash, but said there is a clear process to follow whenever a driver is involved in a collision.

"If you are involved in an accident, we review that. There is refresher training, you are pulled off the road, you have to go out and be reassessed and make sure there are no issues," he said.

He said the goal is to avoid future collisions.

"We have a process that is really rigorous. We review what occurred and we want to make sure it doesn't reoccur."

A truck tows the damaged bus from Westboro station on Saturday. The double-decker struck a shelter the day before, killing three people and injuring 23. (Idil Mussa/CBC)

Impaired driving ruled out

Ottawa police have ruled out impaired driving as a possible cause of Friday's crash.

Diallo was arrested at the scene for questioning and later released without charges.

The condition of the bus-only roadway is one possible factor police collision experts are examining. Some passengers reported the bus appeared to skid before slamming into the shelter overhang.

"We'll see if there was ice and if it did or didn't have an impact on the collision," Police Chief Charles Bordeleau said Monday CBC Radio's Ottawa Morning.

Officers with the collision unit used drones to survey the scene from above and have performed a re-enactment of the crash, Bordeleau said.

Among their next steps: gaining access to the bus's event recorder — similar to an airplane's "black box" — and scouring any on-board video captured at the time of the crash. Police have filed warrants to obtain that data.

Bordeleau said there could also be a coroner's inquest. He promised a "thorough, independent investigation," dismissing suggestions that one city agency investigating another could be seen as a conflict.

Ottawa Mayor Jim Watson said Monday he has full confidence in the police investigation, and said early speculation about the cause of the crash is unhelpful.

Watson also expressed his full confidence in the city's fleet of double-decker buses.

On Monday, Transport Canada confirmed its motor vehicle collision investigations team is "working closely" with the Ottawa police and OC Transpo "to provide every possible support during the investigation."