There was still a streak of light in the sky at 8:50 p.m. April 29 when a Korean teen-ager named Hye Ja Yi stepped off a bus at a busy Gaithersburg intersection and began walking toward her home, about three-fourths of a mile away.

The route to her family's home wound along West Deer Park Drive, an area of luxury brick town houses, climbing red roses and children on bicycles. Hye Ja, who turned 18 on March 15, should have been able to walk it on this balmy spring evening in a few minutes.

But she never got home.

Her battered and partially clothed body was found at about 6:30 the next morning in a ditch behind the construction site of a Giant Food Store at Muddy Branch and Fields roads, about two miles from the Yi home.

Hye Ja ("Hey-jah") Yi had been sexually assaulted and then stomped to death, Montgomery County police said.

"I have investigated 20 to 25 murders in which victims were killed with knives, guns, all sorts of weapons," said Sgt. Miles E. Alban Jr., who is in charge of the Yi case. "But I have never seen anyone stomped to death before."

The mysterious slaying, still unsolved, has stunned the peaceful community of Gaithersburg, the high school where she played piano in the school orchestra and the tightly knit Korean community. Yi's parents, who came to Gaithersburg from the suburbs of Seoul in 1978 seeking a better and more secure life for their four children, are emotionally shattered.

"All the laughter is gone from the family," said Dennis Adams, Yi's uncle.

Frank Masci, principal at Gaithersburg High School, where Yi was a junior, said that the tragedy has touched all of the 2,000 students.

"The kids see that they are mortal . . . that they are vulnerable," he said. Counselors from the Bethesda Crisis Center have been working with Yi's closest friends to help them cope with their loss, Masci said. The school also sponsored lunchtime sessions with self-defense information for interested students, he said. In addition, Yi's classmates have raised more than $800 for a memorial for her.

The Baptist minister who had known Yi as a regular member of the Friday evening and Sunday morning activities at the Korean Baptist Church, 310 Randolph Rd., Silver Spring, said that the Korean community was first shocked and then grieved by the slaying. An estimated 10,000 Koreans live in Montgomery County.

"Now they are angry," said Won Ki Kim. "They are asking why this happened . . . especially when Koreans came here looking for more safety, more happiness. When this happens, you have to reevaluate the reasons for coming here."

Members of the business community in Gaithersburg also have been disturbed by the slaying.

"They are alarmed -- not panicked, but concerned -- by what has happened," said Montgomery County Police Chief Bernard Crooke. "They don't want this kind of violence in our community. They want the person found. They have put up a substantial amount of money as a reward."

The Gaithersburg and Upper Montgomery Chamber of Commerce have donated $1,000 to the police department's Crime Solvers reward fund to be paid to the person who provides information leading to the arrest of the killer or killers. Chris' Restaurant in Gaithersburg, where Yi and her family have worked for years, donated $2,000 to the fund.

Family and friends of the girl have offered an additional $10,000. (The police telephone number to call with information is 840-2444 in Montgomery County.)

Those who knew Hye Ja Yi say she was an unlikely target for such violence. The slim young woman, who was just over five feet tall and weighed less than 100 pounds, is remembered at Gaithersburg High School as an average student who worked hard at her music and her studies.

"She was a good kid; she never caused any trouble," said Masci, the school principal.

Susan Fritts, the manager of Petite Street, the store in Lakeforest Mall where Hye Ja had an after-school job, described her as a "great person to work with. You say, 'Can you do this?' and she says, 'Sure, I can do that,' and she did and there was no argument."

Kim said that Hye Ja "is the last person we'd imagine this would happen to . . . . She was such a quiet, gentle girl."

The slaying has been especially troubling to those who have watched Hye Ja and her family struggle to improve their lives since arriving here seven years ago with few belongings.

"They weren't penniless," said Adams, who is married to Hye Ja's aunt. "But they only had what they could carry in their bags and blankets."

Adams, who sponsored the family's move to the United States, helped the Yis, their three daughters and one son settle into an apartment in Gaithersburg in June 1978. Hye Ja, the third of the four children, was 11 years old. She and the other children were enrolled in Montgomery County schools, where they quickly learned English and adapted to the American system of education.

Min Chae Yi, 49, the father, now works in the stockroom of a Gaithersburg electronics firm. But to supplement his daytime job, the whole family has worked evenings at Chris' Restaurant at 201 E. Diamond Ave.

"They do everything," said Chris Valanos, the owner. "They cook, wash dishes, clear tables, whatever needed to be done."

Within three years, the Yis had managed to buy the brick town house on West Deer Park Drive. They also bought a car. Helen, the oldest child, now 22, enrolled in the University of Maryland to study music. Hye Sook, now 19, found a job and launched a study of fashion design. John, 15, is a junior high school student.

About three years ago, when Hye Ja was 15, she had open heart surgery, Adams said.

After recovering from that, she returned to school, to after-school work at the restaurant and to her piano lessons. On April 23, one week before she was killed, she participated in the competition sponsored by the Maryland State Music Teachers Association. She was awarded a grade of A minus for her performance, according to the certificate on file at Gaithersburg High School.

A close friend said that Hye Ja hoped to enroll in a music school in New York after graduating from high school next year.

Hye Ja was scheduled May 4 to take the Scholastic Aptitude Test, a prerequisite for college-bound students. But all the hopes for Hye Ja Yi were snuffed out the night of April 29.

She attended school as usual that day and then reported for work at the Petite Street store at the mall at 5:30 p.m.

She helped straighten merchandise that had gotten messed up in a markdown sale, helped tape a box, assisted shoppers, and, when business slowed, cleaned out the employes' refrigerator.

By 8:30 p.m., there were so few customers in the store that Fritts told Hye Ja that she could leave for the day, even though the store was not scheduled to close until 9:30.

Hye Ja usually got a ride home with a relative or a friend, Fritts said. But on this night, Hye Ja's brother was ill and the family apparently was unable to provide a ride, Fritts said.

Hye Ja scooped up her schoolbooks, her purse and a pair of pink slacks she had purchased from the Merry-Go-Round shop and rushed to catch the bus that runs south along Frederick Avenue (Rte. 355).

She was wearing a beige blouse, purple slacks and gold-colored high heel shoes and was carrying a brown vinyl purse and pink plastic shopping bag.

Police say the bus driver later told them that she rode to West Deer Park Drive, got off the bus and started walking down the street.

Ten hours later her body was discovered by construction workers at the site about three miles away.

Her purse, shopping bag and books were missing, police said. But she was wearing a Gaithersburg High School ring engraved with her name.

Investigators checked with the school and found that Hye Ja Yi was enrolled there as a junior. By 2:30 p.m. police had a positive identification.

On the night of Wednesday, May 1, officers set up a roadblock on West Deer Park Drive, about halfway between the bus stop where Hye Ja had disembarked and her home. Two uniformed officers and seven detectives interviewed an estimated 100 motorists between 8:30 and 9:30 p.m., asking them if they had noticed anything on the night of Monday, April 29. The roadblock interviews were repeated the following Monday, May 6.

Investigators knocked on doors looking for leads. Over a week they contacted residents of an estimated 400 homes.

James H. Elkins, commander of the police department's Germantown investigative section, said that police worked 360 overtime hours incident in the 10 days following the slaying, conducting interviews, sifting through tips and trying to find out who killed Yi. Normally there are no more than 10 overtime hours for a 10-day period, he said.

Thus far they have published composite pictures of two men who were seen in the area of the slaying and who may have information that could help police solve the case, Elkins said.

While police pressed on, friends and family gathered for a memorial service May 1. Kim, who conducted the service in Korean and English, said that the church service was attended by about 500 persons, including classmates, relatives and working associates of the family.

Ellsworth Briggs, the director of Hye Ja's school orchestra, remembers the service as an extremely emotional experience. "The family was very distraught," he said. "It was terrible. I shudder to think about it."

At Gaithersburg High School, student emotions have run the gamut from grief to anger, even though Hye Ja was not widely known outside a small circle of friends, principal Masci said.

"Students are asking who would do this," he said. "They are thinking the person who did it must be crazy and that they're not safe because this crazy person must still be around."

As a result, parents are urging their children to be more cautious -- and the children are listening. "My mother has told me not to walk alone," said Rosemary Shay, a dark-haired senior who is active in the Student Government Association. Shay said she has heeded the advice.

In an effort to help students channel their emotions, Tony Deliberti, the adult sponsor of the class of '86, recommended that they raise money for a memorial.

Deliberti told students: "We have cried. We have sorrowed. Now let's do something so she will be remembered."

The response has been enthusiastic. To raise money, students have been selling candy, soliciting local businesses for contributions and staging car washes.

Deliberti said he found 50 $1 bills tucked into a book on his desk with an unsigned note:

"The note said that the students had had a car wash and wanted the proceeds to go for Hye Ja's memorial," he said