A deadly rockslide at Glacier National Park is prompting a warning about potential dangers for tourists. A family of five was driving along a popular Montana road when their car was hit by falling rocks, killing a teenage girl inside.

Pat Cummings said she was driving down the road in Glacier National Park when traffic stopped amid sounds of horror.

"I could clearly hear a woman crying frantically, screaming, you know, 'Please help her. Someone please just help her,'" Cummings said.

Park officials confirm a car driven by a family of five from Utah was caught in a rock fall that generated enough debris to fill the bed of a pickup truck. Some of the stones were a foot in diameter.

"Whether it's rain, whether it's wind, you know, all of these things can cause rocks to fall," said Lauren Alley, a spokesperson for the park. "But rockfall in this magnitude is very unusual."

A 14-year-old girl in the car was killed. Her parents and two siblings were injured.

"If there had been more cars probably closer together, I do think it could have been worse," Cummings said.

The rockslide happened on the 50-mile long "Going-to-the-Sun Road," driven by about 2 million people every year. The road's last rockfall death was more than two decades ago -- and the danger is not always preventable.

"It's a little bit tough to inspect the mountain above the road," Alley said. "Rocks are continually shedding as part of that geologic process."