The owners of Britain's Alton Towers say they don't know when the amusement park will reopen after four people seriously injured in a crash between two roller coaster cars.

The ambulance service says men aged 27 and 18 and women aged 19 and 17 suffered serious leg injuries when a car carrying 16 people on the Smiler ride crashed into an empty, stationary car on Tuesday. It took rescuers several hours to remove the passengers, who were stuck 25 feet (8 meters) above ground.

?Chief executive Nick Varney of Merlin Entertainments, which owns the park 150 miles (240 kilometers) north of London, said Wednesday that an investigation was underway. He said a supposedly fail-safe measure to stop two cars crashing "didn't work the way it was supposed to."

One roller coaster car slammed into another Tuesday on one of Britain's biggest amusement park rides, leaving four teenagers seriously injured and stranding passengers 25 feet (8 meters) in the air, officials said.

A car on the Smiler ride carrying 16 people crashed into an empty, stationary car at the Alton Towers amusement park a little after 2 p.m., the West Midlands Ambulance Service said. It said two teenage boys and two teenage girls suffered serious leg injuries.

The ambulance service said the four were being treated at the scene and would be transported to trauma centers by helicopter. It said the other 12 people onboard were not believed to be seriously hurt.

Musician and radio broadcaster Danny Simm tweeted that he saw "people unconscious, knocked out. Blood everywhere. It really was shocking."

Alton Towers, 150 miles (240 kilometers) north of London, is one of Britain's most popular tourist attractions. The park said there would be "a full investigation once all the guests have been evacuated, which is our priority."

A platform was built to help paramedics and firefighters reach the riders, who were stuck in their seats at roughly a 45-degree angle.

Visitors to Alton Towers said the Smiler closed down earlier in the day because of technical problems, but had reopened before the crash.

The Smiler is billed as the world's first roller coaster with 14 loops. Alton Towers says it features "a series of twisted psychological effects including optical illusions, blinding lights and near misses designed to mess with your mind."

It opened in May 2013, two months later than scheduled, and has twice been shut down because of technical or mechanical problems.