Hunters walking through Northern Virginia's Bull Run Mountains Saturday discovered the wreckage of a single-engine airplane which investigators say probably had crashed in early December, instantly killing its two occupants.

Federal officials said yesterday they are uncertain when--or why--the Beechcraft plane crashed, spilling Christmas presents over a heavily wooded mountainside in Fauquier County about 35 miles west of Washington.

"Right now I can't tell you if it occurred on Dec. 1st or 2nd," said Gene Sundeen, a National Transportation Safety Board investigator from Atlanta. He said the plane apparently had sufficient fuel for its planned flight from Jacksonville, Fla., to Watertown, N.Y., and added that his agency was checking weather conditions for Dec. 2 to determine if bad weather might have caused the accident.

"On the 1st of December there was less than two miles' visibility in the area," he said. "We're trying to collect data for the 2nd to see what the weather was like then."

Officials had begun a ground search for the plane on Dec. 3 but halted it six days later after failing to find the aircraft. Killed in the crash were 56-year-old Kenneth E. Wade, town manager of Adams, N.Y., and Doris Cooper Vanwaldick, 31, of Middleburg, Fla. Virginia state police said Wade was the pilot of the plane, which had been reported missing Dec. 1.

"It was pretty well broken up," one of the hunters, Rodney Krone, 40, of Philomont in Loudoun County told the Associated Press. "It hit trees first and came down into the mountain."

Krone, who was hunting with his 13-year-old son, Michael, said they were checking to see if the son had hit a deer when they came upon the crash. "It was kind of an eerie feeling. I saw a white thing which happened to be the tail section. I came upon the fuselage and saw a hand sticking up and thought "Oh, my God . . . ' "

State Trooper Alvin E. Hampton said relatives of the passengers indicated that Vanwaldick was flying to New York to have a "pre-Christmas" celebration before returning to Florida. "I guess the Christmas gifts were intended for some of the relatives up there," he said.

James Beyer, chief medical examiner for Northern Virginia, said a preliminary autopsy report found that Wade and Vanwaldick died from "multiple internal injuries" to the chest and head. Federal officials said yesterday they will issue a final report in about 90 days.