CAMBRIDGE -- Ontario's police watchdog is investigating following a collision in Cambridge that involved members of Guelph police and sent two people to hospital with serious injuries Thursday night.

The three-vehicle collision happened around 6:30 p.m. Thursday in the area of Kossuth and Chilligo Roads.

According to Waterloo regional police, the collision involved two members from Guelph police. The members were in two separate vehicles.

As a result of the crash, the Special Investigations Unit (SIU) has invoked its mandate.

The SIU told CTV News a civilian vehicle heading in the opposite direction as the officers collided with the first police cruiser and then struck the second.

The driver of the civilian vehicle and driver of the second policer cruiser were taken to hospital with serious injuries.

The SIU says the 63-year-old man driving the civilian vehicle is in critical condition. Guelph police say the officer's injuries are non-life threatening.

John Adams, who works nearby, believes he arrived at the scene shortly after the crash happened.

"Terrible, catastrophic," he said. "The engine was blown apart. It was liquid everywhere. It was smoking because the electronics were still hooked up."

Adams said he helped the civilian after the collision.

"I was able to cut the seatbelt off the gentleman in the car, he was not coherent, but I spoke to him anyway," Adams said.

Nearby resident Ray Anderson said he heard sirens Thursday night and saw three ambulances, multiple police cars and two fire trucks arrive.

"It went on for hours," he said. "I knew something serious happened because they usually clean it up and get traffic going."

Kossuth Road was blocked for most of the night.

The SIU has assigned three investigators, two forensic investigators and one collision reconstructionist to the case. The SIU investigates all cases of serious injury involving police.

Anyone with information or video evidence relating to the crash is asked to contact the SIU.