A 28-year-old Fairfax County man was found dead Saturday in a remote wilderness river in an Alaskan national park where he was on an extensive solo hiking and rafting trip, authorities said yesterday.

Peter Kysar, whose father described him as an avid hiker since age 12, was found floating in the White River, a glacial river in Wrangell-Saint Elias National Park and Preserve, just east of Anchorage, said John Quinley, a spokesman for the National Park Service in Alaska.

Although the cause of death has not been determined, Quinley and family members said they suspect Kysar's raft capsized in the frigid waters of the White River, where he would have suffered from hypothermia and then drowned.

Authorities had been searching the rugged terrain for Kysar since Monday, when family members from Oregon alerted them that Kysar, an aerospace engineer at the Institute for Defense Analysis, was overdue on a two-week journey in a park known for having the highest concentration of 14,000-foot mountain peaks on the continent.

"He does have extensive backpacking experience," said his father, John, by telephone from Alaska, where he had flown to join the search. "The only thing that was new to him on this trip would have been rafting in the extremely cold waters around here."

John Kysar said his son had been planning the 130-mile journey for months. "He was going to raft down the White River and cross the border into Canada," his father said. He had alerted his family to expect him on June 25 and was reported missing to Alaskan authorities three days later, Quinley said.

Peter Kysar, who had lived in the Lincolnia section of Fairfax for about five years, is the second Northern Virginia resident to die in the wilds of Alaska in the last year. In September, 24-year-old Christopher J. McCandless, of Fairfax County, was found starved inside an abandoned bus where he had sought shelter while hiking solo outside Denali National Park.

Quinley said last week's search involved 20 people on the ground, three helicopters and three fixed-wing aircraft. Searchers combed an 80-square-mile swath of country around the headwaters of the White River where Kysar had been seen by another solo hiker on June 24.

Quinley said the region gets "quite a few" solo hikers. "We tell people it would be safer with another person around," he said.

"The last sighting was not too many miles from where his body was ultimately found," Quinley said. Authorities found Kysar's raft Wednesday evening and found his body about noon Saturday. He had not been wearing a life vest, Quinley said.

The area where Kysar was found is an unusual place for rafters, Quinley said, because it requires them to carry the heavy rubber craft at least 25 miles across rugged terrain. "The trail he walked up is referred to as the 'goat trail,' " said Quinley, who estimated that the raft and other equipment Kysar was carrying weighed more than 80 pounds.

"He probably put the raft in a little early. This area that he was starting in has fairly rough water for a small raft," Quinley said, adding that the waters smooth out farther down the river.

A neighbor of Kysar's said yesterday that Kysar had lived in two homes on Yellowstone Drive and knew several of the residents.

"He was very quiet and very busy," Elizabeth Glowicz said. "He was a nice gentleman."