Sgt Christopher Cook told The Associated Press that police believe the woman fell Friday at Six Flags Over Texas in Arlington, and that there appears to have been no foul play.
Park spokeswoman Sharon Parker confirmed that a woman died while riding the Texas Giant roller coaster, dubbed the tallest steel-hybrid coaster in the world, but did not specify how she was killed.
Some witnesses said the woman who died wasn't properly secured.
"We are committed to determining the cause of this tragic accident and will utilise every resource throughout this process," Parker said in a statement yesterday. "It would be a disservice to the family to speculate regarding what transpired."
Cook, spokesman for the Arlington Police Department, said police, fire and emergency medical services responded to the park around 6:45 pm Friday in reference to a woman who had fallen from a train car while riding a roller coaster. He said the woman was pronounced dead at the scene.
He said the park and the Texas Department of Insurance, which approves amusement rides and ensures they are inspected, will be involved in further investigating the accident.
Carmen Brown told The Dallas Morning News that she was waiting in line to get on the Texas Giant when the accident happened and witnessed the woman being strapped in.
"She goes up like this. Then when it drops to come down, that's when it (the safety bar) released and she just tumbled," Brown, of Arlington, told the newspaper. "They didn't secure her right.
One of the employees from the park, one of the ladies, she asked her to click her more than once, and they were like, 'As long you heard it click, you're OK.' Everybody else is like, 'Click, click, click.'
"Hers only clicked once. Hers was the only one that went down once, and she didn't feel safe, but they let her still get on the ride," Brown said.
Six Flags said the ride will be closed as the investigation continues.
The Texas Giant is 14 stories high, and has a drop of 79 degrees and a bank of 95 degrees. It can carry up to 24 riders. It first opened in 1990 as an all-wooden coaster but underwent a USD 10 million renovation to install steel-hybrid rails and reopened in 2011.