Four people were killed at an Australian theme park on Tuesday, when a “critical incident” occurred at one of the park’s oldest and most popular rides.

Queensland Police confirmed that four people, all of them adults, were killed after an “incident” occurred on the Thunder River Rapids ride at the Dreamworld Park in Coomera.

Park officials say Dreamworld was closed at 3:30 p.m. local time, and guests directed to leave as soon as possible.

The ride in question is billed as a “family ride”, with small children over the age of two permitted to ride.

The four victims of Tuesday’s “incident” include two men and two women, all in their thirties and forties. Authorities say there were no additional injuries.

While few details of the accident have been released, the ride reportedly suffered a “malfunction,” trapping two of the four victims, while hurling the other two.

In a written statement to the media, police say the accident involved a conveyor belt.

“Police were called to the site at Coomera around 2.20pm following reports that a number of people had been injured by a conveyor belt.”

According to the Gold Coast Bulletin, one of the ride’s circular rafts, which seats six people, flipped over on the conveyor belt.

Australian Prime Minister Malcolm Turnbull spoke briefly on the accident, calling it a “very, very sad and tragic event.”

“Our thoughts and prayers are with the families of those who lost their lives. We trust there will be a very thorough investigation into the causes of this incident in the days that follow.”

Australian police closed the amusement park until further notice and are investigating the event.