(CNN) -- A lone backpacker was killed in a grizzly bear attack while hiking in Alaska's Denali National Park, the National Park Service said Saturday.
The first sign of trouble came Friday afternoon, when three hikers noticed an abandoned backpack and signs of a struggle -- including torn clothing and blood -- along the Toklat River, the park service said in a news release.
The hikers went back to a rest area, about three miles to the south, and alerted authorities around 5:30 p.m. Friday.
About two and a half hours later, park rangers conducting an aerial search spotted at least one grizzly bear and, after touching down, the unidentified victim's remains.
The body remained at the site Friday night after rangers decided to wait until Saturday morning to remove the remains because of the waning light and the number of bears in the area.
The park service said that, based on "initial evidence," authorities believe a bear attacked the backpacker by the river and dragged his body to a more sheltered, bushy area.
The area of the Denali backcountry where the attack occurred has been closed -- prohibiting all hiking and camping in that area -- "until further notice," the park service said.
About 12 grizzly bears have been living this summer around where Friday's attack occurred, the park said, citing wildlife biologists.
Grizzly bear attacks are not common, though they are not unprecedented. Last September, a grizzly attacked and killed a hunter in northwestern Montana within sight of another hunter. Earlier that year, grizzly bears killed two men in Yellowstone National Park, according to the park's superintendent.
This week's attack is the "first known bear mauling fatality" recorded in Denali National Park and Preserve, according to the park service.