A roller coaster crash left four people seriously injured in the U.K.’s Alton Towers theme park on Tuesday.

Alton Towers Roller Coaster Crash Seriously Injures 4

The accident occurred Tuesday afternoon on The Smiler when one car carrying 16 people collided with an empty car that was left on the tracks at a speed of about 20 mph. Witnesses described hearing what sounded like a car crash when the roller coaster cars collided on the track, and watched in horror as the 16 people on the ride were left dangling on the track, screaming for help.

Witness Scott Holden filmed a short video on his phone in the direct aftermath of the accident. Holden wrote on YouTube that he had been taking a photo of the empty car when the crash occurred. The video shows the two cars, now fused together, swinging on a curved piece of track as the momentum slowed. Guests on the ride can be heard screaming, “Stop this f—king ride!” and “Help us!”

The area was quickly evacuated, and the cars stopped moving, stalling at a 45 degree angle about 25 ft. in the air. It took rescuers four hours to evacuate all those in the crash, as they first had to build a platform to reach the area safely.

Four people were seriously injured and airlifted out to receive treatment. The two men, aged 27 and 18, and two women, 19 and 17, reportedly suffered “significant lower-limb injuries.” Others suffered neck and head injuries, and eleven people required medical attention.

“The women’s injuries were worst, both suffered open wounds and damaged legs; the two men had leg and chest injuries but were less seriously hurt,” said senior paramedic Peter Howell.

Two victims of the crash, Joe Pugh, 18, and his girlfriend Leah Washington, have been publicly identified, but it is unclear whether or not they are two of the four who were seriously injured.

Alton Towers theme park closed during the rescue effort on Tuesday and will remain closed through Thursday; though it is possible the park may extend the closure as officials complete their investigation.

According to reports, the empty car was put on the track for testing after The Smiler stalled earlier that day. Mechanisms of the rollercoaster are meant to prevent two cars being on the same stretch of track at the same time, and investigators are currently looking into why the system didn’t stop the ride automatically.

“Those two cars should not have been on the same piece of track. Technically that should not have happened. There are braking locks that should stop two cars being on the same section of track and somehow that didn’t work the way it was meant to,” said Nick Varney, CEO of Merlin Entertainment, which owns Alton Towers.

Alton Towers’ divisional director, Ian Crabbe, also released a statement, saying, “May I first say that my colleagues and I are devastated by what has happened here today, and our only priority at this time is to work with the emergency services to get the 16 people affected off the ride safely, and most importantly those who are injured are being cared for. Our concern is with and for them and for their safety, and all our thoughts and efforts are focused on them.”

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