Witnesses are describing a "gruesome" scene in the wake of a five-vehicle crash on Highway 417 in Ottawa's western outskirts Sunday.

The collision is being investigated by Ontario's Special Investigations Unit because one of the vehicles had been the subject of a police pursuit earlier that day.

"Whatever the most gruesome thing you can imagine, that was yesterday," said Ottawa Senators broadcaster Dean Brown, 57, who was travelling to Peterborough with his partner, Susan MacDonald, 46, and her 75-year-old mother, Linda MacDonald, when their Dodge Durango SUV became involved in the violent collision.

It was as if a few bombs had gone off. There's pieces of cars and vehicles everywhere. - Dean Brown

Brown said a black car was speeding eastbound in the westbound lanes of the highway when it ran head-on into two vehicles. One of those slammed into Brown's SUV, forcing it off the road where it came to rest near a rock wall.

The driver of the black car and the driver of another vehicle died in the crash. Two men in their late 20s who were in another vehicle suffered serious but non-life-threatening. Others suffered minor injuries.

Ottawa Senators broadcaster Dean Brown was travelling with his partner, Susan MacDonald, and her mother when they saw a black car slam head-on into another vehicle. 2:02

'Carnage'

Brown said he had to crawl out the passenger door to escape the wreckage. What he saw next was "carnage," he said.

"It was as if a few bombs had gone off," he said. "There's pieces of cars and vehicles everywhere."

Brown said it didn't appear the driver of the black car attempted to avoid the crash.

"He never tried to take the shoulder. He never tried to miss anybody."

Brown counts himself fortunate to walk away with bumps, bruises and a few sore ribs. He said he was still picking shards of glass from his arm on Monday.

Linda MacDonald, a retired nurse, helped care for one of two men injured in another vehicle, Brown said. He credits the first responders who soon arrived on the scene for their work helping other victims.

Dean Brown, right, and Susan MacDonald, left, escaped the collision with minor injuries. (Judy Trinh/CBC)

'Horrific' crash

Alex Freedman came upon the scene minutes after it happened. He was on his way to Sudbury, Ont., and chose to take the 417 instead of his usual Highway 7 route.

"It was a horrific crash," Freedman said. "There were many vehicles spread on the road and on the shoulder of the highway."

One of the five vehicles involved in Sunday's head-on collision on Highway 417 near Panmure Road. (Submitted)

He said he saw several adults being treated on the highway, but one of the things that touched him the most was when he spotted a child's car seat on the ground.

"Anytime you see something like that it hits pretty close to your heart," Freedman said.

Paramedics said no children were injured in the crash.

Alex Freedman says he saw a child car seat amidst the wreckage. Paramedics say there were no reports of children injured in the crash. (Submitted)

Police pursuit called off

Ontario Provincial Police said officers had been pursuing a suspect vehicle, but called off the chase before the crash.

Audio obtained from Broadcastify, an online service that provides access to emergency dispatch calls, recorded the moments before and after the collision as OPP officers considered a "rolling block" to stop the suspect's vehicle.

"I was just advised of a massive head-on collision, wrong way ... just west of Panmure," a dispatcher says.

Brown said he saw no police vehicles in pursuit of the black car before the crash.