The Red River Ex in Winnipeg has reopened a roller-coaster after a teenage boy was critically injured on the Crazy Mouse ride Thursday evening.

Garth Rogerson, the CEO of the Red River Ex, said investigators told him the 16-year-old boy had scaled a1.8-metre wire fence after exiting the ride to try to retrieve a hat. Police have not confirmed that information.

Staff at the Red River Ex help passengers stranded on the Crazy Mouse roller-coaster after an accident involving a 16-year-old boy. (Katie Nicholson/CBC)

Emergency crews and police rushed to the fairgrounds just outside of Winnipeg around 6 p.m. after receiving calls that a person had been injured.

Witnesses reported seeing a boy lying on the tracks, injured and covered in blood.

Witness Kara Pruden said immediately after the accident, staff put up purple covers to hide the boy and told people nearby to back up.

Rogerson confirmed a teen boy was taken to hospital in critical condition after an accident on the ride.

The ride was stopped immediately after the incident, and nearly an hour later staff were seen helping people stranded in cars get off the second tier of the roller-coaster.

Fairgoer Debra Ginter's four children were on the ride at the time of the accident.

"I was terrified. It was scary, and I thought it was one of my kids," she said.

Ginter's four kids were trapped on the ride for over an hour. Ginter's son Johnathon said he saw the aftermath of the accident.

"The cart stopped, and I heard a lot of people screaming, and I looked forward, and I saw blood," Johnathon said.

Johnathon's sister said she saw blood pouring from the teen's head.

No mechanical failure, CEO says

Rogerson said there was no mechanical failure on the roller-coaster at the time of the accident, and all rides are inspected daily.

He said the accident happened after the boy exited the car at the end of the ride.

Worker injured on ride in 2006 In 2006, a worker was injured on the Crazy Mouse roller-coaster at the Red River Ex in Winnipeg. Provincial investigators found the worker left his control station and went into a restricted area of the ride while it was moving. Investigators concluded there were no mechanical or structural faults with the ride.

"He was on the ride and he lost his hat, and we would retrieve it for him, but he didn't wait and he jumped the fence to retrieve it," said Rogerson.

Rogerson said the roller-coaster can go up to 45 km/h, and hats are forbidden on the ride. He also said the fence was in place to protect riders.

"There's a reason why we have that fence there, and if you jump it, then bad things happen," he said.

Provincial investigators would only confirm the boy was injured in an area that is "off limits to both the public and ride attendants while the ride is in operation."

A provincial official said the investigation is continuing, but they do not believe there was any mechanical failure on the ride.

They are still working to find out how the teen was able to reach the area where he was injured.

Rogerson said his staff responded appropriately and all of the necessary safety measures were in place at the time of the accident.

The ride was immediately shut down for a full safety inspection, which it had passed by early Friday afternoon. The ride was running without any people on it by around 2 p.m. It began loading passengers shortly after.

Rogerson said employees are being offered counselling.

Winnipeg police are now investigating the incident, and reported Friday afternoon that the boy is still in critical condition.

Long Plain First Nation Coun. Barb Esau told CBC News the teen is originally from Long Plain, near Portage la Prairie. She said his friends and family are praying for him.