Four people were killed Tuesday in an accident at a popular theme park on Australia’s east coast, police and the park said.

Police said two men and two women died while on the Thunder River Rapids ride at Dreamworld. The theme park said it did not know how the accident occurred and that an investigation was underway.

"Dreamworld is working as quickly as possible to establish the facts around the incident and is working closely with emergency authorities and police to do this," the park said in a statement. The park was closed following the accident.

The Thunder River Rapids ride whisks riders in circular rafts along a fast-moving, man-made river. Early reports indicate the ride's conveyor belt malfunctioned, forcibly ejecting two of the victims and pinning the other two either under water or in machinery.

The victims were in their 30s and early 40s, said Gavin Fuller, an emergency worker with the Queensland Ambulance Service.

Lia Capes, a witness, told the Australian Broadcasting Corp. she was just about to go on the ride when she saw people running out, crying.

"I was speaking to one of the guys and he said it was the raft or the boat thing in front of him, the whole thing flipped and everyone was screaming," Capes said.

"This is a very sad day for our city," Gold Coast Mayor Tom Tate said in a statement. "Our thoughts are with the families of those affected — and the emergency staff in attendance."

Australia's Prime Minister Malcolm Turnbull said there would be a thorough investigation. "Theme parks are a place for family fun and happiness, not tragedy," he told reporters.

Dreamworld is located on Australia's Gold Coast, a region but also a city in the state of Queensland. It is an area known for its beaches, surf spots and is popular with vacationers. The Great Barrier Reef sits off the coast of Queensland.

Matthew Centrowitz, an American Olympian and track-and-field athlete, tweeted that he had been on the ride immediately before the accident.

"Dude! Just got off a ride 15 mins ago that has 1 person seriously injured & 2 others trapped. #Dreamworld," the athlete's tweet said.

The Thunder River Rapids ride is considered to be family-friendly and is open to children as young as two. An incident in 1984 at the Six Flags Great Adventure amusement park in New Jersey in which eight teenagers lost their lives when they were trapped by fire in a haunted house is considered to be one of the deadliest accidents at a theme park.