Aspen Hill is scared.

Shortly before 9 o'clock one night late last month, a man wearing a hood forced open a living-room window of a house in the normally quiet Silver Spring neighborhood and grabbed an 18-year-old woman, forcing her into her dining room where he raped her, according to police.

Three nights later on Oct. 2, about a mile away, a 13-year-old girl was attacked as she lay on a recreation room sofa watching television about 11:15. She also was raped, apparently by the same man, police said.

Those rapes, and 14 others verified by police, have terrorized the upper-middle-class neighborhood of about 500 homes in Montgomery County for more than a year.

"It's a very tense situation," said a mother of two daughters, a teen-ager and a toddler, who lives on a street where two of the rapes have occurred. "It's all people talk about nowadays. Neighbors who never used to speak are now calling each other to talk. We've talked about the rapes so often that I've had to try to explain to my toddler what a sexual assault is. It's absurd that we should even have to discuss it in the first place."

Police, who say they are no closer to solving the cases today than they were a year ago, characterize the rapist as a person with "an exceptional mind" who studies his victims and carefully plans every aspect of the assaults. He may even be someone the victims and their families know, police say.

The residents living in Aspen Hill's $120,000 to $140,000 homes -- high-level government employes, real estate agents, airline pilots, college professors -- are doing everything possible to secure their four square-mile area. Several families have installed spotlights that burn all night. Others have installed heavier door locks and sophisticated security systems. Still others have chopped down some of their favorite trees and shrubbery to reduce the number of hiding places near their homes.

Afraid those measures won't be enough, the residents are also in the process of forming a neighborhood watch program.

Some women, once afraid only for their daughters, now fear for themselves as well. A middle-aged mother of teen-aged sons said she used to jog through the community after dark, but now is afraid to leave her house alone even to go to the local shopping center.

"I feel someone could be watching me at all times," she said. "It's a shame to think that if I want to walk next door to share recipes with a neighbor, I've got to call ahead and say, 'Hey, Martha, I'm coming over. Look out for me!' "

One day recently, after her children had gone to school and her husband to work, the woman said she settled into a tub for a hot bath. Then she realized she could not hear activities in other parts of the house with bath water running. So she got out of the tub, went downstairs and got on the phone.

"The knob on my front door turned," she said, "and I ran out of the house through the garage."

When she turned to look back toward the house, she saw her husband standing at the front door. He had come home ill for the first time in 13 years.

"I was really frightened," she said. "If I'd had a gun, there could have been an awful accident."

On Oct. 1, the day before the most recent attack, the Strathmore/Bel Pre Civic Association sent out a bulletin urging families to take special precautions. One family thought they were already secure, having installed locks a month before on what they thought were all their windows and doors. The next night, they let their 13-year-old daughter stay home alone for the first time.

A man broke into the house through a second-story window, where apparently a lock wasn't installed. He then forced the girl to go to an upstairs room and get him some money, then ordered her downstairs to the den where she was raped.

Police said all of the rape victims have been between the ages of 11 and 38 years old, and that most are blindfolded or have their heads turned so they cannot see their attacker. Medical authorities have verified all the rapes, police said.

Police believe there's only one rapist because the victims all describe him similarly, and because he operates in the same manner each time. They say he is a white male between 18 and 30 years old, who wears gloves to avoid leaving fingerprints and a stocking or ski mask to avoid being identified. He strikes between dusk and midnight. He often talks to his victims during the attacks and tells them details of their personal activities, police said, and he sometimes apologizes when the sexual assault is over.

He also has usually planned his route of escape and sometimes leaves a nearby door ajar so he can make a quick exit, according to police.

"He leaves no clues," said Montgomery County police detective David A. Hutchison, who said he is among several investigators working on the case full time. "He is definitely familiar with the area. We believe he either lives here, once lived here, or has friends in the area.

"The person we're looking for is the kind of person that no one suspects. To the people who know him, he probably appears to be the all-American boy next door."

Montgomery County police, who have no composite sketch of the rapist, say they now patrol Aspen Hill more frequently, although they won't say how often. Residents say police are stopping and questioning boys and young men fitting the suspect's general description whom they see walking around the neighborhood after dark.

But it is young girls, seldom seen alone outside their homes now, who have felt the greatest impact of new security measures.

"We feel like prisoners in our homes," said one 15-year-old girl. "I'm afraid, but mostly angry that I can't stay at home alone or that I can't have friends over. When I come home from school, I stay outside until someone comes out to get me. Even if I have a key, I don't go inside, because he could be in there.

"When we're driving around the neighborhood and we see a young boy in a car, we take down the license number because that could be [the rapist]," she said. "Even when I'm inside the house, if I'm at one end, I keep thinking that he could be coming in at the other end."