(CNN) -- Search crews fanned out across the vast backcountry of the Great Smoky Mountains National Park on Friday after severe thunderstorms the night before killed two people and injured eight, park officials said.
An unknown number of hikers and campers may have weathered the Thursday night storm on the dozens of trails and isolated primitive camping sites in the most hard-hit western portion of the park, spokesman Carey Jones said Friday morning.
"We have no idea how many people are in the backcountry," he said. "We're just now getting people onto trails."
Officials have no indication that anyone was hurt or is in distress in the most isolated portions of the park, which straddles the border between Tennessee and North Carolina. But the park's backcountry has no cellular telephone service, and the storm hit during a busy time, noted Chief Ranger Clayton Jordan.
Jordan identified the dead as Ralph Frazier, 50, of Buford, Georgia, and 41-year-old Rachel Burkhart of Corryton, Tennessee.
Frazier, who was on a motorcycle, died when a tree limb fell and struck him on the head, Jordan said. A passenger on the motorcycle was not injured, Jordan said.
Jordan said Burkhart died when she was struck by a tree at a popular swimming hole near the park's Abrams Creek campground.
She was apparently among a number of swimmers who were scrambling to get out of the water when the storm roared through and toppled a tree that knocked them back into the water, Jordan said.
Three members of another family were also injured in the incident, including a 7-year-old girl who was trapped under water by the tree, Jordan said. Bystanders performed cardiopulmonary resuscitation and revived the girl, Jordan said.
She and her father -- who sustained serious injuries including back injuries, broken ribs and a collapsed lung -- were flown to the University of Tennessee Medical Center in Knoxville. The girl's mother was also injured, but less seriously, and was taken by ambulance to the same hospital, Jordan said.
Their conditions were not immediately available.
Meanwhile, authorities were working to evacuate campers who survived the storm without injury but were stranded by fallen trees, Jordan said. Those campers chose to stay behind overnight while others were led out by vehicle caravan through an emergency route cut through fallen trees, Jordan said.
About 38 people spent the night in a Red Cross shelter, he said.
Hundreds of motorists were stranded throughout the park by fallen trees, Jordan said. Many had been rescued, he said.
Forty miles of roads in the park remained closed Friday due to downed trees, Jones said.
The storm, which struck suddenly during the busy Fourth of July holiday week, stressed the ability of park rangers and outside emergency crews to respond, Jordan said.
"To have such a wide swath of the park at a very busy time of the year to be impacted by such severe storms is highly unusual," he said.
Hundreds, perhaps thousands, of people were visiting the park when the storms hit, Jones said.
The area near Cades Cove, an isolated valley that offers some of the best wildlife viewing opportunities in the park, was one of the hardest hit by the storm, Jones said. The area, which is accessible by only one road, remained closed to visitors Friday. Several other park roads and campgrounds also remained closed, Jordan said.
Park officials have been too busy with rescue efforts to assess damage to park facilities, Jordan said.
In addition to the park, the storm also caused extensive damage to a marina and a hardware store in Sevier County, Tennessee, according to John Matthews, the county's emergency management agency.
The county had to evacuate its juvenile detention center when part of the hardware store's roof blew onto the building, he said. No one was injured, he said.
In Blount County, the storm caused no injuries but knocked down many trees, emergency management director Bart Stinnett said.
The storm also knocked out power to 56,000 customers of the Knoxville Utilities Board in Knoxville, Tennessee, just north of the national park, the utility said in a statement.
Power outages around Knoxville were down to about 13,000 as of 11 a.m. Friday, according to the utility. Crews were coming from Ohio to help in the around-the-clock effort to restore power, which could take several days, the utility said.
The weather that hit the area began as a cluster of thunderstorms that formed along the Ohio River moving south. The storms continued to intensify as they got into more heat and humidity, and converged on the foothills of the Smokies.
Straight-line winds of 70 mph were recorded.
Great Smoky Mountains National Park, which straddles the Tennessee and North Carolina border, is the most-visited national park in the United States. Between 8 million and 10 million people visit the park each year, according to the National Park Service.
The park covers more than 800 square miles in the Southern Appalachian Mountains and boasts a diversity of plants and animal life, and is known for its beauty.
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CNN's Rob Marciano and Mariano Castillo contributed to this report.