ARLINGTON — As horrified passengers watched Friday, a woman fell to her death from the towering Texas Giant roller coaster at Six Flags Over Texas. Witnesses later reported that she might not have been properly secured.The womans identity has not yet been released. Witnesses said she basically tumbled over, perhaps when the ride made a sudden turn, and disappeared into the evening.Investigators were expected to be on scene today to try to determine what happened. The park opened as scheduled this morning, although the ride remained closed.John and Darlene Putman of Rockwall said they were in line when the Giant returned and saw an agitated man and woman in their 20s get out.They were screaming, My mom! My mom! Let us out, we need to go get her! John Putman said.Ride operators spoke with the couple, and one made a call. They put on helmets and headed for an area under the coaster, the Putmans said.The accident occurred shortly after 6:30 p.m., said Sharon Parker, spokeswoman for the amusement park. Our thoughts and prayers are with the family and friends during this difficult time, she said. One witness told The Dallas Morning News that she was next in line for the ride and watched as workers fastened the woman into her seat. One of the employees from the park one of the ladies [the victim] asked her to click her more than once, and they were like, As long you heard it click, Youre OK," Carmen Brown of Arlington told the newspaper.Everybody else is like, Click, click, click. Hers only clicked once. Hers was the only one that went down once, and she didnt feel safe, but they let her still get on the ride. That could have been me.Brown said the woman basically tumbled over.Another witness, Kanisha Howell, told CBS 11 that she and her daughter watched the coaster rise from their place in line."When it got in the air and started coming down, I don't know if she had a seizure or what, but she fell out of the cart and just fell out of the sky, Howell said. Six Flags has released little information about the accident. Arlington police referred all questions back to amusement park officials. The parks planned performance by Bridgit Mendler as part of its summer concert series was postponed, according to the parks official website. Park visitors David Leal, 18, and Halee Rice, 17, of Farmersville said they waited 45 minutes before riding the Texas Giant earlier in the evening because the coaster was undergoing testing. It was unknown whether the delay was related to the fatal accident.Originally opened in 1990, the Texas Giant reopened in 2011 after a $10 million renovation. The ride rises 14 stories high and boasts a 79-degree first drop, the steepest in the world for wooden roller coasters, a bank of 95 degrees and a speed of 65 mph.Fridays death was the parks second involving a guest. In 1999, a 28-year-old Arkansas woman died and 10 others were injured when a boat capsized on the Roaring Rapids ride.In 2006, nine park patrons were injured on the Texas Tornado when one of the mechanical bearings that spins the ride malfunctioned.In the 1960s, a park worker died when the El Sombrero ride was accidentally turned on as it was being worked on.