An investigation aided by a citizen's discovery of a tree house containing stolen items has led to the arrest of nine juveniles and two adults and, police say, the solution of nine burglaries, one attempted burglary, four instances of vandalism and an arson in the Folkstone subdivision near Reston.

Fairfax County police said all the incidents occurred last month. They said the vandalism consisted of firing a gun into occupied houses and the arson caused an estimated $50,000 damage to a recently completed but unoccupied home.

The tree house in a wooded area of the subdivision was discovered by a woman whose home was burglarized when she and her family were on vacation last month on Cape Cod.

Returning home on the Labor Day weekend, she said, she discovered several items missing: a coin collection, a watch, a tape recorder, Little League baseballs, a BB gun and her husband's .22-caliber pistol. The electricity had been turned off. A green moustache was drawn on a picture of her son.

In the tree house, the woman, who does not want to be identified, found her husband's gun and the watch.

Following additional investigation by police, nine neighborhood juveniles ranging in age from 14 to 17 plus a 19-year-old and a 20-year-old were arrested.

Capt. R. L. Ellis of the Chantilly police substation said the adults, Mark Lewis of 10648 Maple St. and Stephen W. Trenor of 11926 Central Dr., and four juveniles were charged with the burglaries, three other juveniles were charged with the four instances of vandalism and two other juveniles were charged with arson.

Lewis and Trenor are being held in lieu of $500 and $10,000 bond, respectively, and all of the juveniles have been released in custody of their parents pending a Juvenile Court hearing. Ellis said the parents have been given the names of those burglarized or vandalized so that restitution can be made.

Fairfax police officer George Stevens, who has been investigating the incidents, said he would not characterize the neighborhood suspects as "a ring," because in most cases the thefts were not made "for the specific intent to sell."

Stevens said he does not know why the youths in the subdivision became involved. "I don't think anyone knows," he said.

"These kids have everything," said the woman who found the tree house.

Alan Jacobs, who has built 120 homes in the subdivision of $100,000 houses, and several residents of Folkstone said the initials "FGM" have been painted on some walls in the community, and that they stand for "Folkstone Guerrilla Movement."