The quickest roller coaster in the world has been suspended until further notice, after multiple customers reported broken bones from the ride.

Since December, at least six riders sustained bone fractures after riding “Do-Dodonpa,” a roller coaster that goes at “super death” speed in the country’s popular Fuji-Q Highland Park, the park’s operator said. Four of them said they broke their neck or back, a spokesperson for the park told VICE World News.

The incidents, alerted to authorities on Aug. 17, are baffling officials.

Built in 2001, the ride goes from 0 to 180 kilometers (or 112 miles) per hour in 1.56 seconds, making it the quickest roller coaster in the world. But the park said it was the first time riders broke their bones on the ride since it went into operation two decades ago.

In 2017, the ride was even modified to bring the top speed from 172 kilometers per hour to 180, but the park said there were no reports of serious injuries, including bone fractures, until December.

No technical issues were found upon initial investigation, according to Fuji-Q Highland. The ride’s manufacturing company, Sansei Technologies, apologized to the injured customers but said it also didn’t know what caused the injuries.

Roller coaster rides that result in severe injury are rare. The last roller coaster-related death in the country was reported in 2007, when an axle on a car broke during a ride in Expoland in Osaka and sent the roller coaster crashing into a guardrail.

Naoya Miyasato, an architecture professor from Nihon University who studies roller coaster designs, said accidents that result in broken bones are unheard of. “Roller coaster designs must all abide by government-approved standards, so the fact that there are multiple similar accidents is unusual,” he told VICE World News.

Although the Japanese government hasn’t found the reason for these injuries, Miyasato said it could likely be an issue with the roller coaster’s rapid acceleration. At its peak, the ride’s acceleration is more than three times the force of gravity—that’s comparable to the G-force experienced by astronauts during a rocket launch.

“If a rider can’t withstand the acceleration, then they sustain injury, which could be what’s happening here,” he said.

But in addition to the coaster’s rate of acceleration, Miyasato said that the way riders sit may be an issue.

“If they detected no serious concerns with the actual ride, then it could be the way people were sitting. But if a person was sitting incorrectly, say with space between their backs and their seat, it’s the responsibility of the park employees to check their seating position,” he said.

Much like most high-speed roller coasters, “Do-Dodonpa” requires riders to lean back against their seat and wear over-the-shoulder restraints, keeping as little space as possible between their back and the backrest.

According to Mainichi Shimbun, one of the riders who reported injury said she may have been sitting forward during the ride.

Statistics from the International Association of Amusement Parks and Attractions show that the likelihood of being seriously injured on a roller coaster in a U.S. amusement park is about one in 15.5 million rides.