The 33-year-old car driver was badly injured in the crash, but she was tested for alcohol when she arrived at a hospital.
"A blood sample was taken from the driver and we will have the results soon and we will be able to see if the driver will face any criminal charges," Montreal police spokesperson Jean-Pierre Brabant said.
Officers investigating the accident also believe one of the vehicles involved may have run a red light. Many witnesses on the street say that the bus driver had the right of way.
Police are also looking into whether the bus was speeding.
The crash occurred around 6 p.m. Tuesday at 55th Avenue and Lindsay Boulevard in Dorval, in an industrial area in the western part of Montreal. In what an official for the transit drivers' union called "something that's beyond belief," the bus driver was ejected from the bus, which tipped over in the accident, crushing him underneath.
The union identified him on Wednesday as 49-year-old Sylvain Ferland, whose wife and sister are also drivers for Montreal's transit corporation, the STM. (The union initially reported his age as 52).
A 61-year-old woman who was in the BMW's passenger seat also died. The BMW driver is still in hospital in critical but stable condition.
In total, two people died and 11 people were injured in the crash, two critically.
Ferland was driving a bus on Route 196, which runs from the Côte Vertu metro station south to Lachine.