Two dozen people who got stuck near the top of a roller coaster at a
"All 24 patrons on the ground being evaluated by EMS," Prince George's County Fire Chief Marc Bashoor said on Twitter.
The Joker's Jinx coaster stalled, and riders were helped off with support from the county Fire/EMS Department, Six Flags America said in a tweet.
A video stream on ABC News showed rescuers using ladder trucks to reach the 24 riders, many of whom were holding umbrellas to protect themselves from the sun.
"We're working on harnessing up all the patrons" so the safety bars in the ride's cars can be let up, Assistant Fire Chief Paul Gomez told the Los Angeles Times in the middle of the operation. He said the harnesses would be attached to the roller coaster track, the fire truck's ladder or another fixed point.
The cars were about 75 feet from the ground, he said.
Gomez emphasized that the rescue operation was focused on safety. "We could certainly get up there quicker if we had someone injured" and lower people down with a rope if necessary, he said, but "we would rather take our time and do it safely."
He said it took a while to access the riders because of the cars' location in the middle of the ride. Officials moved some fences and other objects on the ground so the ladder trucks could reach better positions, he said.
The roller coaster includes "a cobra roll, a sidewinder loop, a corkscrew, and countless swift reversals," according to Six Flags America's website, but the cars were stuck on a stretch of track in an upright position.
The theme park did not answer calls seeking comment.
Last month, a derailment left 22 people stuck aboard the Ninja roller coaster at Six Flags Magic Mountain in the Los Angeles area. The riders were stranded for nearly three hours before they were rescued.