A double-deck city bus smashed into a transit shelter during rush hour on Friday. Ottawa authorities said three people were killed and others were 23 injured.

Police Chief Charles Bordeleau said the bus driver was arrested after the crash, but he declined to provide details on why she has been detained. He said police were still investigating the cause of the crash.

"It is with deep sadness that I report there were three confirmed casualties, two of which were on the bus and one that was on the platform," Ottawa Mayor Jim Watson said at a news conference, Reuters news agency reports.

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Ottawa Hospital's trauma center said it had received nine patients "in critical condition" from the crash.

First responders at the crash scene in Ottawa on Friday, Jan. 11, 2019. Justin Tang/The Canadian Press via AP

The accident happened at the Westboro station in a neighborhood west of downtown Ottawa. Photos posted on Twitter shows extensive damage to the front of the bus, with the roof of a shelter pushed well into the bus's second deck.

Canadian Prime Minister Justin Trudeau expressed his condolences on Twitter.

I want to send my deepest condolences to the families of victims and everyone affected by today’s tragic bus crash in Ottawa. Thank you to the first responders who rushed to help - we’re monitoring the situation closely. — Justin Trudeau (@JustinTrudeau) January 12, 2019

The bus has a capacity of 90 people, both sitting and standing. The crash snarled afternoon traffic as emergency crews descended on the scene.

In 2013, an Ottawa bus collided with a Via Rail train in suburban Barrhaven, killing six people.