The province's police watchdog is investigating a crash in Brampton on Thursday that left a 37-year-old woman and her three young daughters dead.

The four-vehicle crash occurred near Countryside Drive and Torbram Road shortly after noon.

Peel Regional Police Const. Akhil Mooken said when officers arrived on the scene at around 12:15 p.m., they found several people suffering from various injuries.

A witness at the scene told CP24 that one vehicle was driving very fast through the intersection when it collided with another car.

"He just smashed the car," she said. "My husband went over there and took the kids out."

She added that a police cruiser was following behind that speeding vehicle at the time of the collision.

The Special Investigations Unit (SIU) has been called in to probe the circumstances surrounding the crash.

Based on preliminary information, the incident began when a Peel Regional police officer who was travelling west on Countryside Drive observed an Infiniti on the eastbound lanes, SIU spokesperson Monica Hudon told reporters at the scene on Thursday evening.

A short time later, at the intersection, the Infiniti collided with a van that was travelling north on Torbram Road, Hudon said, adding that the van subsequently hit a light pole.

Inside the van were the mother and her three daughters – a six-year-old, a four-year-old, and a one-year-old. Hudon said the six-year-old was pronounced dead at the scene.

The mother and her four-year-old daughter were rushed to Brampton Civic Hospital where they later died, she said.

The one-year-old was also transported to a Brampton hospital, but was later transferred to the Hospital for Sick Children, where she died of her injuries.

The family was from Caledon.

The driver of the Infiniti, a 20-year-old man, was also transported to a hospital where he remains in critical condition.

Two other vehicles were also damaged during the collision. Hudon noted no other serious injuries were reported.

"Understanding what transpired once the officer observed the Infiniti is part of the SIU's investigation," Hudon said.

"It's still very early on in the investigation and determining that will be very important in the coming days."

When asked if the Infiniti ran a red light, Hudon said it will be part of the investigation, as well as whether the officer's lights and sirens were activated.

She said the SIU will conduct a thorough investigation as quickly as possible.

Four investigators, three forensic investigators and one collision reconstruction investigator have been assigned to the case, Hudon said.

Mooken would not confirm if police were in pursuit of a vehicle at the time of the crash but said no arrests have been made at this time.

The SIU is an arm's length agency that is called in to investigate any incident involving police and a member of the public that results in death, serious injury, or an allegation of sexual assault.

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The area is expected to be closed for several hours for the police investigation.

"I think easily we are expecting this road closure to go well into the evening hours, just due to the scope of how many vehicles, the incident scene, and the fact that this is now going to be an SIU investigation," Mooken said.

"We are appealing for witnesses. We know this is a heavily populated area. There is a lot of businesses in the area as well as residences. If anybody did witness this and has not yet had the opportunity to speak with investigators, we are asking them to call us."

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Cause of 'devastating' crash still under investigation

Mooken said while the SIU conducts its probe, Peel Regional Police's major collision bureau will be doing a parallel investigation into the cause of the crash.

"We will consider all factors in this investigation," he said.

He added the collision has been devastating to all of those involved, including the officers who responded to the scene.

"This is devastating, devastating incident... it is shocking," he said.

"As a parent, when I first saw this call come across, the first thing I could think of was my own daughter. (I) tried to imagine the pain that family was going to be facing."