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Four people received minor injuries at Six Flags Magic Mountain when the Ninja roller coaster derailed after hitting a tree, stranding 22 riders some 20 feet off the ground, fire officials said Monday evening.

Los Angeles County Fire Department first responders were called to the scene just before 6 p.m., according to a supervisor with the department.

UPDATE: State Safety Investigators Respond to Magic Mountain After Ride Derails

Initial information indicted as many as 30 passengers were on the ride at the time of the accident. That number was later reduced to 22.

Multiple cars holding riders appeared to be stuck at an angle in an area where the tracks pass under trees, aerial video from the scene showed.

"We were riding on the Jet Stream when we saw the Ninja go into the trees," witness Brandy Chambers said. "It was a lot of leaf noise and then cracking noises ... and a lot of screaming."

The situation was caused by a tree branch that fell onto the track and obstructed the train, according to Magic Mountain spokeswoman Sue Carpenter.

"It looked like a pine tree fell and it fell across the track of the Ninja ride," said L.A. County Fire Inspector Rick Flores.

"The car came around the corner and it hit the tree," he said.

The impact caused the first car to derail, Flores said. It was the passengers riding in the first car that were injured.

Firefighters appeared to be cutting at or struggling with foliage near the large white pillars that support the ride.

Just before 8 p.m. rescuers began bringing the stranded riders to the ground, aerial video showed.

All of the passengers were said to be alert and talking with park personnel during the evacuation process, Carpenter said.

The two injured riders were transported to a local hospital as a precautionary measure, Carpenter said.

One of the riders complained of neck pain, the second reported suffering a knee injury, fire officials said.

The ride would remain closed until a "thorough inspection of the area is complete," Carpenter said.

The ride is described as a "stealth assassin" and "suspended swinging roller coaster" on the Six Flags website.

"Shoulder harnesses secure you in place but don’t expect to just sit in one place—you’re going to be swinging all over, a full 90 degrees each way," the site's description reads.

KTLA's Nidia Becerra Tijerino and Kennedy Ryan contributed to this article.

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