Authorities in Yosemite National Park were trying to determine why a large tree limb fell early Friday, killing two children sleeping in their tents.
Park officials released few details about the accident, which occurred about 5 a.m. in the Upper Pines Campground in Yosemite Valley.
But witnesses described a grim scene at the campgrounds when the branch fell.
"I heard this loud bang and then a woman screaming at the top of her lungs," camper Daniel Moore, told the San Francisco Chronicle. "I knew something was not right. I stepped outside to see what was going on and saw a lot of people clustered around their campground. It made me sick to my stomach when I figured out what had happened."
Authorities described the victims as minors but did not disclose their identities or any other personal information.
When Yosemite rangers responding to 911 calls arrived at the campsite, the campers were dead.
"Our thoughts are with the families as they grieve this tragedy," park Supt. Don Neubacher said on Twitter.
Yosemite spokesman Scott Gediman said the branch was from a black oak. There were no winds and rangers are unsure why the limb fell, but "fallen branches like this one are a common occurrence across the park," Gediman said.
California's persistent drought has killed more than 12 million trees in forest lands across the state. But Gediman said it was too early to tell if parched conditions played a role in Friday's incident.
Upper Pines is the largest of three reservation campgrounds in Yosemite Valley, which at this time of year is packed with visitors. Located near Curry Village and shaded by tall conifers, the campground remains open.
The park is closing another campground, Tuolumne Meadows, for five days next week to treat rodent burrows after finding two dead squirrels infected with the plague. Officials called it "an extremely precautionary public health measure" in the wake of a recent plague case.
A Los Angeles County child contracted the disease, which is spread by flea-infested rodents, during a July family trip to Stanislaus National Forest and Crane Flat Campground in Yosemite. The disease was treated with antibiotics, and the child recovered.
Public health investigators found plague-carrying fleas on several squirrels and chipmunks trapped at Crane Flat, which was closed for four days for flea treatments. At the Tuolumne Meadows Campground, two dead golden-mantled ground squirrels tested positive for the disease, said Dr. Vicki Kramer of the California Department of Public Health.
"Plague is endemic," Kramer said. "It's been in California since the 1900s," she added, and "the risk of exposure to people is very low."
Fatalities occur every year in Yosemite, one of the most heavily visited national parks in the country. They typically involve climbing or hiking incidents in the rugged Sierra terrain. Two BASE jumpers died in May after they leaped from an overlook above Yosemite Valley and hit a ridge.
But over the years falling tree branches have also taken a toll.
Two tourists were killed and nine were injured in 1985 when a 25-foot oak branch fell 15 feet onto an open-air tram carrying 50 passengers.
A similar accident occurred in 1992, when an oak branch landed on a tour bus, injuring seven people. Three years ago, a concession worker died when a tree limb crashed onto his tent cabin during a windstorm.
Video from ABC30-TV showed the large branch that fell. The area, which including several tents, was blocked off by yellow police tape.
Moore, the witness, told ABC30 the branch fall sounded like a gunshot.