A roller coaster at the Six Flags America amusement park in Prince George's County was closed by a Maryland state agency yesterday after riders were injured Saturday night when they were stranded 10 stories in the air and then suddenly jolted back to the ground.

About a dozen riders were treated at the scene Saturday for neck and back problems. Riders were also splashed with fluid when a hydraulic line broke.

It was the second time in a month that a malfunction triggered the ride's automatic safety shutdown.

All rides must be inspected before the park opens for the summer, said Craig Lowry of the Maryland Department of Labor, Licensing and Regulation, which oversees amusement park rides. The department pulled the certificate of operation until it can determine what caused the ride to stall.

The "Two-Face: The Flip Side" ride will remain closed until it can be proved safe.

Six Flags won't begin its investigation until the state inspectors arrive, spokeswoman Karin Korpowski said. The park was busy yesterday, she added. "We have a lot of park to enjoy."

Are the rides safe? " 'Safe' is an interesting word," Lowry said. "It's relative freedom from risk. The commissioner has to issue a certificate saying there's safe operation.

"We're looking at it in a snapshot in time. If the park doesn't keep up with maintenance, the park doesn't do this, doesn't do that, it could change the circumstances."