When the Fire Ball amusement park ride collapsed on July 26 at the Ohio State Fair, one person was killed and seven others were injured. One row of seating split off and crashed near the ride. And now the cause of the accident has been revealed. The arm broke because of excessive corrosion.

The ride was supposed to spin six rows of seats around and around, 40 feet above the ground. While the seats spin, CNN reports, the “the entire structure moves like a pendulum.”

A video of the incident showed the seat split off. Rhonda Burgess, a witness, spoke to CNN. “The ride had four riders per cart. This piece snapped off and the riders came out of the cart,” she said. “At least two (people) flew through the air at least 20 feet before landing on their backs on the concrete.”

The ride is made by a Dutch manufacturer. Albert Kroon, product manager, issued a statement that blamed the accident “excessive corrosion on the interior of the gondola support beam dangerously reduced the beam’s wall thickness over the years.”

The Fire Ball, which makes the rounds from fair to fair, had been in service for 18 years.

The corrosion inside the beam was not visible from the outside. “This finally led to the catastrophic failure of the ride during operation,” Kroon said.

The ride had passed visual inspection. The corrosion, in the middle of the arm, would not have been seen, even with more technical inspection. Industry experts say that only ultrasound scans, which are expensive, would have found the problem.

Tyler Jarrell, an 18-year-old from Columbus, died in the accident. Jarrell had just enlisted in the Marines. The others injured in the incident were either on the ride itself, or struck by flying debris when the ride came apart.

“There were an estimated 30,900 injuries associated with amusement attractions seen in emergency departments in 2016,” CNN notes, citing Patty Davis, press secretary for the US Consumer Product Safety Commission. In the last 7 years, 22 of those accidents have been fatal.