At about 5 p.m. on what had been an uneventful Tuesday last September, Karen Maureen Benson, 15, left her mother's Fairfax County apartment for an unknown destination. "I'll see you later," she told her mother.

The ninth-grader at Groveton High School was so conscientious, according to friends and her mother, Mary, that she almost always told her family where she was going and how long she would be there. Only if she planned to be gone for just a few minutes would she leave without telling her destination.

When Karen Benson left home on Sept. 23, there was no indication that she would never be seen alive again.

Exactly one month later, a 12-year-old boy and his dog were playing in the privately owned gravel pits near the Springfield Forest subdivision and the Richmond, Fredericksburg and Potomac Railroad -- 4 1/2 miles east of Karen's home -- when they discovered the body of a young girl partially covered by underbrush. Although a cause of death has not been determined, a coroner used dental records to confirm yesterday that the body was that of Karen Benson.

"When she didn't call or come home that night, I knew that something had happened to her. I knew that she hadn't run away," said Mary Benson from her Hybla Valley apartment yesterday. "I suspected foul play from the beginning."

Beyond yesterday's identification, however, very little was known about the girl's unexplained disappearance and death. Investigators and police have yet to determine how, when or where she died. Mary Benson's own canvassing of her neighborhood has found no witnesses, and no clues.

"She had just turned 15 [two days before her disappearance]. No one had seen anything. I've called and talked to everybody," Mary Benson said. Fairfax County police spokesmen added that they have been unable to locate anyone who may have seen the girl leave her home, nor have they been able to determine whether she was approached by a stranger.

Karen Benson was wearing a T-shirt, blue jeans and a denim jacket on the day she disappeared, according to a police department press release circulated last week. She was described as 5-feet-3, about 105 pounds, with long brown hair and blue eyes. Police have urged that anyone with information should call investigators at 691-3195.

Her mother described Karen as "a typical teen-ager," with several girlfriends and one boyfriend, a personable girl with a penchant for drawing and writing fiction.