A rock slide along the popular Lassen Peak Trail at Lassen Volcanic National Park has killed a 9-year-old boy from Red Bluff, California, and injured his 13-year-old sister. Crews were trying to determine Thursday what caused the slide and have closed the trail until Friday.

“They were hiking along, it was a family, a mother, father, and three kids," park Superintendent Darlene Koontz said Thursday. "Two of the kids stopped to take a rest, the slide occurred. One of the kids died on the peak, another little girl was injured. And then the last child was fine.”

The trail runs 2.5 miles one-way to the top of the 10,457-foot peak. Much of the mountain is barren, lacking of trees and other vegetation that could help stabilize the flanks. The trail begins at the peak parking area at an elevation of 8,500 feet and zig-zags across this steep and rocky landscape to the summit.

The superintendent said there were no indications that the slide, which occurred late Wednesday afternoon, was human-caused. “All indications are that there was no one else who triggered the slide," she said. "They’re (investigators) going to check with USGS for rumblings, which happen all the time here. There was a huge storm in the area, too.”

The national park is centered around a dormant volcano, one that hasn't erupted since May 22, 1915, but which has a geologically active basement. Lassen Peak is the largest of a group of more than 30 volcanic domes that erupted over the past 300,000 years in the landscape that now holds the national park, according to the Park Service.

There were 50-60 people hiking the trail when the slide occurred, and some came to the family's aid. Three helicopters were used to remove the injured. Along with assessing the mountainside Thursday, rangers intended to question other hikers.