The school year was a week old, but Sofia Silva and her friends were dressed as if they were going to Skateland or Planet Fun instead of junior year at Courtland High. When she climbed into Sheri Moden's red Hyundai that Monday morning last September -- The Day She Came Up Missing, as friends still refer to it -- Sofia wore a loose white ribbed sweater, denim shorts, six rings, a gold-tone ankle bracelet, Nikes and her signature -- lots of purple accessories.

The two teenagers made their ritual stop at the Fas Mart, where Sofia used part of her $10 allowance to buy Cinn-A-Burst gum and a 20-ounce bottle of Dr Pepper, which she would sip between classes. After Melanie Kiker and her twin sister, Angela, dropped her off that afternoon at her house on Bounds Street, Sofia talked to Sheri on the telephone for half an hour, considered going to the Cougars' first football game that night, took her homework and a can of grape soda to her front stoop and was never seen alive again.

The smallest details of the 16-year-old's very normal routine on Sept. 9 are replayed as a community and its police weigh the possibility that the 5-foot-5, 100-pound girl was the first of three victims of a serial killer or killers in Spotsylvania County.

Known in law enforcement circles as "stranger abduction homicides," these killings are so comparatively rare -- less than one-half of 1 percent of all slayings -- that few police departments in the country have the experience to investigate them when they occur.

What began as a single and seemingly successful investigation into Silva's death has now turned into an anxious and at times confused manhunt. The subsequent abduction and slaying of the Lisk sisters, Kristin, 15, and Kati, 12, in May has forced residents in this rural community 70 miles south of Washington to face the terrifying possibility that someone snatched three children from in front of their homes, drove them somewhere and killed them.

These kinds of killings are considered by law enforcement to be among the most difficult to solve. A study released last month on child abduction killings, one of the most comprehensive to date, reviewed about 1,000 cases and found that at least a third had never been solved.

But in the Spotsylvania killings, the difficulties faced by any law enforcement agency have been compounded by a botched forensic analysis, the erroneous indictment of a man in Silva's slaying and a delay in exploring links between her killing and that of the Lisk sisters.

By the time the Lisk sisters were killed, Silva's case had been closed for four months, concluded in January by authorities who were convinced they had the right man behind bars based largely on the forensic tests. When they conceded that the tests were wrong and dropped charges in Silva's death early in June, investigators were left with a wide-open case and three victims, while residents of this growing county of nearly 80,000 faced the prospect that a killer had been roaming free all along.

The effect of the killings on a community noted for its Civil War battlefields, one caught between its rural roots and the suburbanization brought by new residents who commute to Northern Virginia, has been profound. At Courtland High School, all adults now wear photo identification. People are increasingly armed, vigilant. What has happened is often hard to articulate, but it is deep and permanent.

"I don't think the kids realize it, but parents do," said Bob Stack, one of Sofia's teachers. "Crimes have been committed against these girls and against the community."

David Henry, the youth minister at Sofia's church, said devout members now struggle with one of their basic tenets: Love even those who hate.

"How," he asked, "do you find the grace of God to do that?" She Was Always Smiling'

Purple.

Sofia discovered it that summer. It was the color of the bands on her braces and of her backpack. Her eye makeup and her jacket were purple. Her fingernails and her toenails were painted purple. Her gifts to others were purple-colored. A normal teenager by all accounts in a community where young people work at Food Lion and Waffle House, Sofia had a passion for things purple that set her apart.

Aside from that, she was an average teenager, friends said, quiet and reserved until she got to know people. She liked boys, hung out at the video arcade and watched comedy and action movies. Her friends tended to be teenagers from the neighborhood. She wanted to be a beautician and had just begun cosmetology classes as part of her high school curriculum.

"Usually she was always smiling," said Jennifer Catron, 15, who attended church with Sofia. "She'd always call me Shorty and just talk about everything, especially boys, which ones she liked, which ones she didn't like." Waiting and Hoping

Phyliss Silva called police at 9:30 p.m. Her husband, Humberto, was out searching. Friends, relatives, neighbors, members of Fairview Baptist Church gathered at the Silva house, where sheriff's deputies began their investigation. One million children are reported missing every year, many of them runaways, but from the onset, few believed that Sofia had left willingly.

A search by hundreds of volunteers and rescue teams, dogs and helicopters yielded nothing. For five weeks, family and friends kept vigil. "We waited. We hoped that she was just out. She'd come back. She never did," said Melanie Kiker, 18. On Oct. 14, workers breaking up a beaver dam 20 miles east, in neighboring King George County, found a body wrapped in a light blue-green moving blanket and bound with rope.

The autopsy could not determine the cause of death because the body was decomposed, but the medical examiner said it probably was strangulation, asphyxiation or stabbing, Commonwealth's Attorney William F. Neely said. Silva's body was clothed -- the medical examiner noted the faded bluejean shorts and a stained, long-sleeved sweater. Her shoes, socks and underwear were missing, and her pubic hair had been shaved.

By December, Spotsylvania sheriff's detectives had narrowed their list of potential suspects to a handful, among them Karl Michael Roush, 44, an itinerant house painter who had had minor brushes with the law. Roush, who rented a basement room seven houses from the Silva home, had drawn the attention of investigators even before the body was found. Fitting the Profile

Roush's landlord, Charles Hudson IV, told police that Roush would sit in his van and watch neighborhood children, Hudson said in an interview. He also told police that Roush's behavior had changed after Silva vanished -- he stopped drinking beer and took to Jack Daniel's whiskey. Two weeks after the disappearance, Hudson said, Roush abruptly moved out, leaving some of his belongings behind.

Hudson's account fit the profile of a potential suspect, down to the sudden shift in behavior and address. The description helped police obtain search warrants and collect fiber evidence from Roush's van and his former residence.

Detectives then questioned people who knew Roush, including his former wife, Sharon Holm, 34. In a recent interview, Holm said that police asked her whether Roush had ever battered her or shaved her pubic hair and that her answer to both questions was no. While police developed their case against him, Roush, apparently unaware that he was being investigated, moved to Florida to start a business, Holm said.

Not long after Roush left, authorities announced publicly that they wanted to question him in connection with the Silva case. Holm said she called him and told him to return to Spotsylvania. Roush packed his belongings and drove back immediately, arriving around Thanksgiving 1996. Police arrested him on outstanding charges stemming from the shoplifting of a VCR. While he was in the Rappahannock Regional Jail, several inmates told police Roush had claimed responsibility for Silva's death -- an allegation he has denied to his attorneys.

In the state's crime lab in Richmond, meanwhile, the case against Roush was coming together. On Jan. 8, the lab reported that four varieties of fibers found in Roush's van matched fibers found on Sofia's body. "You've got your man; this is overwhelming evidence," a forensic examiner told police investigators, according to Capt. Patricia Sullins, a spokeswoman for the Spotsylvania sheriff's office.

About two weeks later, a grand jury indicted Roush on charges of abduction with intent to defile and murder, largely on the basis of the fiber analysis. Police closed the case even though one key element, the DNA analysis of samples taken from Sofia's vaginal area, did not conclusively implicate Roush. The analysis showed that if there was only one contributor besides Sofia to the DNA samples recovered, then Roush could not have been it. But if there were multiple contributors in addition to Sofia, he could have been one.

Child abductors seldom work in pairs -- the typical profile drawn by studies is that of a sole white male in his twenties -- and Spotsylvania sheriff's officials speculated that the foreign DNA could have come from consensual sex with a boyfriend before the abduction.

But that is not the Sofia friends knew. Several friends said she had not been having intercourse and did not have a boyfriend. Roush's defense team maintained that the DNA results cast doubt on the case. Yet, protesting his innocence, Roush remained in jail. Roush declined to comment for this story.

On May 1, the Lisk sisters disappeared.

A community still shocked by Sofia's death once more cycled through raw emotions: disbelief, dread and finally, despair.

As word of the missing girls spread, the community again mounted search parties and blanketed the area with fliers. But this time, the FBI and state police assigned special agents full time, at the request of authorities in Spotsylvania. In the community, the horror over Silva's killing was now heightened by the fate of the Lisk sisters, whose bodies were found in a river 40 miles south of their home. Although police insisted that there was no link, evidence in the Lisk case would soon cast more doubt on that assertion.

Investigators now acknowledge that they did not immediately identify what they later said could have been a "signature" left by a killer on the bodies of Sofia and the elder Lisk girl: Both had had their pubic hair shaved. They knew Sofia's pubic hair had been completely shaved, and they believed that it was done by the killer. But investigators acknowledge that this similarity was not immediately apparent in the slaying of Kristin Lisk.

According to a more complete account now provided by investigators, the detective present at the Lisk autopsy on May 7 noted that her pubic hair had been partially shaved, but after questioning Kristin's family and friends, authorities were unable to figure out whether she shaved herself.

FBI investigators also did not immediately believe that the same killer was responsible, primarily because the Spotsylvania sheriff's office still maintained that Roush killed Sofia -- and he was in jail when the Lisks vanished.

"They had the suspect for the Silva case, which was Roush, and if Spotsylvania says they're very confident that he is the person for Silva, then there's no way he could have committed the Lisk case," FBI spokeswoman Mary Johlie said. "You pretty much go on what the other department tells you." An Apparent Signature'

It was not until about two weeks after the Lisk sisters were found, investigators now say, that detectives received evidence that caused them to focus on similarities in the cases. The FBI crime lab reported that some of Kristin's pubic hair had been found inside her shorts, causing investigators to conclude that the shaving had been done by the killer, said Sullins, the spokeswoman for the Spotsylvania sheriff's office.

"The big question was, Was this something done by Kristin?' " Sullins said. "It was not until the receipt of the report two weeks later that we learned it was not self-inflicted." Johlie, the FBI spokeswoman, would not discuss why the analysis of Kristin's shorts took two weeks, citing an ongoing investigation.

The apparent signature left on the bodies added to an already long list of similarities. The dead girls were all about the same age; they were slender and had dark hair; they vanished after school from in front of their houses; and there were no witnesses. Moreover, police suspect the killer removed Sofia's underwear and bra and the bra from Kristin Lisk -- what experts in this kind of killing call "trophies."

Meanwhile, an expert hired by Roush's defense team, former FBI lab examiner Harold A. Deadman Jr., reviewed the work of the state's crime lab.

During his May 8 review, Deadman cited discrepancies between state forensic scientist Robin G. McLaughlin's notes and her conclusions and questioned the scientific techniques used in the tests. Deadman said the lack of a clear match in the DNA also made him question the match in fibers, noting that such cases often involve one suspect who sexually assaults a victim.

As part of a broadening investigation, Spotsylvania detectives went to the state crime lab in Richmond on Friday, May 30, retrieved all the Silva evidence and delivered it to the FBI lab in Washington. But Deadman's questions hadn't set off the wholesale review. Sullins, the spokeswoman for the sheriff, said detectives independently were weighing various possibilities and wanted the FBI lab to look at all the evidence. One possibility was the existence of a second suspect in the Silva case, Sullins said, a person who may have acted in concert with Roush.

FBI analysts examined the evidence over the weekend. On Monday, June 2, FBI officials called Spotsylvania authorities and the Virginia crime lab with unsettling news: The four varieties of fibers that had linked Roush to Sofia appeared to have been analyzed incorrectly.

On June 9, FBI lab officials met in Washington with Virginia lab officials and authorities from Spotsylvania to go over discrepancies. McLaughlin and a Virginia lab official agreed that the fibers did not match. Within hours, Spotsylvania authorities announced that the key element of the case against Roush did not tie him to the crime and that they would drop the charges.

The surprise announcement had profound implications: For the first time, police acknowledged that Silva's killer could still be at large and could also have been responsible for the deaths of the Lisk sisters.

"I would have bet my life these cases were not connected," Sullins said. "I was standing out there saying there was no doubt in my mind Karl Roush was responsible for the Silva case. My first reaction {to the lab error} was, Color me with egg on my face,' because I had been so adamant."

At the crime lab in Richmond, the investigators suddenly became the investigated. A review of McLaughlin's lab notes revealed conclusions at odds with her findings. The notes listed properties of the four types of fibers collected from Roush's van and those collected from Silva's body. The conclusion said they matched. But the notes clearly showed significant differences in the fibers' scientific properties.

Paul B. Ferrara, the director of the state's Division of Forensic Science, said he asked McLaughlin how the error could have occurred. She couldn't explain it, and Ferrara said he asked for her resignation.

Next up at the inquiry was McLaughlin's supervisor -- whose name has not been disclosed publicly. He told Ferrara that he had not reviewed her work in the Silva case and that it wasn't his responsibility. The supervisor was demoted to a lab examiner.

"I've never seen work this bad from any examiner," said Ferrara, who has been in charge of the state labs since 1985 and began his career as an examiner in 1971. "We could not explain this in any other way except examiner error. The high profile of the case only made matters worse."

McLaughlin declined a request for an interview. Common Apparel Fibers

But even if McLaughlin had not erred in her work, Ferrara now says, the fiber evidence would have been weak. The fibers were so common that it would have been hard to establish that those found on Silva's body necessarily had come from Roush's van. Moreover, Ferrara said, most fibers did not match.

"The fiber evidence is not the kind of stuff that is going to make a case," Ferrara said. "Even if Robin's fiber results were correct . . . you're talking about hundreds of common apparel fibers, four of which match. A jury's going to convict on that? I don't think so."

Spotsylvania sheriff's officials say they put great weight in the fibers largely because McLaughlin allegedly told detectives that the evidence against Roush was overwhelming and that they had the right suspect in Roush. Sullins said the lab examiner also told authorities that the fibers were unique because they had been exposed to particular cleaners, a conclusion that is not supported by McLaughlin's notes, Ferrara said.

Although police said they had circumstantial evidence implicating Roush beyond what they were told by Roush's landlord, court papers make clear that the fiber evidence was the backbone of the case.

Dropping the case against Roush has not created an outcry, but for him and his family, the ordeal has left lasting scars. Holm was kicked out of a car pool. The couple's 8-year-old daughter, shielded from the news by her mother, learned of the allegations when she turned on the television and her father's face flashed on the screen. The Color of Grief

The weather is not kind to the makeshift memorial at Sunset Memorial Gardens. In Lot 144D, in a section known as the Garden of Devotion, the ink on the notes left by Sofia's friends runs with the rain. On Tuesday, when she would have turned 17, the impromptu memorial included a birthday cake made of foam and covered with flowers.

Purple is now the color of grief. There are purple ribbons on the grave and in front of the house on Bounds Street. Friends said the angel Sofia's mother wears is wrapped in purple ribbon. The Silvas' new car, a purple Pontiac Grand Am, has license plates that read: 4SOFIA.

Now police await the comparison of DNA and other evidence that they expect will tell them whether Sofia's killer is also the killer of Kristin and Kati. And the community, too, waits. "Now we still don't know who it is," said Sheri Moden, Sofia's friend. "It's kind of hard not knowing if we're going to walk out the door one day and someone's going to be saying, I'm the one who did this. Now it's your turn.' " CAPTION: THE SILVA CASE With police now investi-gating the possibility of a serial killer in Spotsylvania, the sequence of events in the Sofia Silva murder takes on new importance. KEY EVENTS SEPTEMBER 1996 Sept. 9: Sofia Silva disappears from her Spotsylvania County home. OCTOBER 1996 Oct. 14: Silva's body is found in King George County, 20 miles east of her home. Oct. 14 (approximately): Charles Hudson IV reports to police that his tenant, Karl Michael Roush, had been watching children in the Silva neighborhood and that after Silva disappeared, he started drinking hard liquor and then abruptly moved out, leaving behind some belongings. NOVEMBER 1996 Nov. 28: Authorities obtain a search warrant to collect fibers from Roush, one of several warrants obtained in this case. JANUARY 1997 Jan. 8: Virginia crime lab's preliminary report on fiber evidence reports four matches between fibers taken from Silva's body and those found in Roush's van. Jan. 21: Roush is indicted by a grand jury on charges of abduction with intent to defile and murder. MAY 1997 May 1: Kristin and Kati Lisk disappear from their Spotsylvania County home, about eight miles from Silva's house. May 6: Lisk sisters' bodies found in Hanover County, 40 miles south of their home. May 7: Spotsylvania sheriff's officials assure the public there is no connection between the Lisk killings and Silva's death, a message they would repeat over the next several weeks. May 8: Roush defense expert examines Virginia crime lab test results and tells lab officials he suspects errors. May 21 (approximately): FBI reports pubic hair found in Kristin Lisk's shorts, which leads authori-ties to conclude that she had been shaved by the killer. Silva, too, had been found shaved. May 30: Spotsylvania sheriff's officials take fiber evidence from Silva's body and from Roush and his van to the FBI lab in Washington for comparison. JUNE 1997 June 2: FBI crime lab officials call the Virginia crime lab to report possible errors in the fiber analysis in the Silva case. June 9: Virginia crime lab officials meet in Washington at the FBI lab, and officials of both agencies agree that the first analysis was wrong and the fibers don't match. Spotsylvania authorities announce charges would be dropped against Roush and tell the public for the first time that the same person could have killed Silva and the Lisk sisters. June 16: Judge dismisses charges against Roush. WHERE THEY WERE FOUND The Lisk sisters and Sofia Silva lived in Spotsylvania County within about 10 miles of each other. SOURCES: Virginia Division of Forensic Science, Spotsylvania County sheriff's office CAPTION: This image of smiling Sofia Silva is among photos published in memory of the slain teen. CAPTION: A memorial at Sofia Silva's grave includes flowers and a foam cake in honor of the 17th birthday she never reached. CAPTION: A page in the 1997 Courtland High School yearbook is devoted to photos showing Sofia's life. She disappeared Sept. 9, shortly after classes had ended and just after she had talked with a friend about attending the first football game of the season. CAPTION: Sofia had taken her homework to the front stoop of her Spotsylvania County home, above, when she vanished. Her body was found about 20 miles away.