Virginia state police issued a lookout today for a bearded, dark-haired man sought as a possible suspect in the slaying of two hikers whose bodies were found buried in shallow graves on the Appalachian Trail here three days ago.

Described as between 25 and 30 years old, wearing a green fatigue shirt and carrying a hunting knife at his belt, the unidentified man, authorities said, was seen "acting weird" by other hikers near the isolated spot 13 miles from here where the two decomposed bodies were discovered over the weekend.

Investigators said they drew the description from interviews with hikers on the trail as far north as Waynesboro, Va., and as far south as Georgia who passed through the area of the killings around May 19. The site, set in thick mountain woods near the Virginia-West Virginia border 270 miles southwest of Washington, has been sealed off while law enforcement personnel continue to comb the underbrush for clues.

Authorities said they have determined no motive for the killings, the first in six years on the trail, although the couple's backpacks and camera equipment as well as the woman's own sleeping bag have yet to be found.

"They've got a 12-day start on us," said state police spokesman E. A. Duff in Wytheville.

The bodies of Susan Ramsay and Robert Mountford Jr., both 27 and fellow social workers at a center for disturbed youths in Ellsworth, Maine, were found covered with leaves and forest debris about 75 yards from a lean-to trail shelter where they apparently had planned to spend the night.

Ramsay, who was spending a two-week vacation hiking the trail from a starting point at Damascus, Va., 150 miles south of here, had been stabbed more than a dozen times in the chest and back. A plastic garbage bag had been placed over her head and a cotton sleeping bag liner wrapped around her body.

Mountford, who had begun the 2,000-mile trail in Georgia two months ago, was shot three times in the head and was found in his sleeping bag.

The slayings, committed on what is regarded as one of the safest sections of the trail, have spread fear among hikers up and down the East Coast. Even before the bodies were found by a team of 40 rescue workers assisted by tracking dogs, warning signs were posted as far away as the Tennessee border.

Since Saturday, when the first body was discovered, wilderness lovers have been avoiding this section of the trail altogether. Many are waiting at off-trail shelters for possible news of the assailant's capture before continuing their journeys. Others have given up plans for walking the rest of the way to Maine and simply boarded buses for home.

"It's just a little too close for us," said Jim Apperson, a Leesburg stone mason preparing to leave a church-run hospice here and abandon his hopes of completing his hike. "I'd just as soon go fishing."

"It's really upset the whole trail community," said Larry Van Meter, executive director of the Appalachian Trail Conference, a nonprofit organization based in Harpers Ferry, W.Va., that oversees the trail. "It's the worst thing that's ever happened."

Until now, Van Meter said, most violent incidents on the trail had been confined to backwoods areas of Tennessee. In 1975, a Unversity of Wisconsin coed was murdered near Iron Mountain, 20 miles south of Abington, Va., and a local man later was convicted in the slaying. Several rapes were reported around the same area in 1979. A few weeks ago, a couple hiking the trail through the Great Smoky Mountain National Park were beaten and robbed.

"We feel sick," said Charles Parry, a professor at Virginia Tech who directs maintenance of 70 miles of trail in this area. "Murders happen every day . . . but we thought the trail was safe from that kind of thing."

For those who have never spent a night in the woods, it may be difficult to imagine the horror this double slaying has instilled in those who use the trail, which draws 3 million hikers annually.

"I don't know how I'm going to feel being out there in the dark," said Neil Chivington, a coworker of Ramsay and Mountford, as he pondered resuming his trek north. "I may just freak out. Every branch that cracks would scare the hell of you."

For Ramsay, death came only 13 miles from what could have been the end of her journey in Pearisburg, a small Giles County community nestled at the foot of a bluff called Angel's Rest. She had pallned to take a bus from here back to Maine, where her parents, from Aurora, Ohio, intended to join her to celebrate her birthday.

Mountford, an experienced rock climber and former westler, and Ramsay met May 9 at the Methodist Hospice in Damascus, intending to finish the 150-miles hike to Pearisburg by May 21. They spent the next week walking the trail with three companions as it wound through the verdant hills of Southwest Virginia. In the last few days before their deaths, they were seen occasionally by acquaintances. On the morning of May 19, storeowners about 20 miles from here recall Ramsay leaving a small amount of cash for her then-trailing companion.

Several days later, friends who had been further behind them stopped to rest at the same shelter where Ramsay and Mountford were killed, unaware that the bodies of their companions lay hidden in the underbrush just a short distance away.

Some said later they noticed that the floorboards of the shelter were smeared with coals from a campfire, apparently to cover bloodstains. Others arriving at the hospice here compared notes and found that the shelter's log book was missing by the time others arrived.

Chivington, who arrived on the scene a few hours before the first body was found, was one of those to see the weathered note Mountford had posted for Ramsay the night of their deaths.

"I can't find Wapiti," he said of an older shelter located on a trail spur nearby. "If I can't find Wapitu [a second shelter built last year], I'll put up my tent."

Ramsay and Mountford ended the day's hike in the narrow valley beneath the peaks of Brushy Mountain and Sugar Run Mountain. In the confusion of many interesting paths and logging trails there, they may have crossed Dismal Creek, a trout stream swollen by recent rains.

Only last year the trail had been relocated away from a road popular among fishermen, hunters and local youths. The older shelter there, the one Mountford could not find, had become a popular gathering place for beer parties.

Ramsay and Mountford eventually found Wapitu, the newer shelter, and bedded down under oaks and Virginia pines. They were one-quarter mile from the nearest four-wheel drive track, but perhaps not alone.

"I was just wondering if you're safe anyplace," said Duane Dempsey of Boston, who drove around the fatal section of trail but was determined today to finish his hike home. "But I can't crawl under a rock. It's just a shame more of us weren't with them. We might have been able to help."