GAINESVILLE, FLA. -- Among the usual clutter of hand-lettered notices on a University of Florida bulletin board was one from "Stephanie" advertising her 8-month-old dog as "gentle and loving," free to a good home.

The note has been amended recently, in red ink, with a more compelling sales pitch: "VERY PROTECTIVE!"

The messages may seem contradictory. But college students here are dealing with far greater contradictions these days.

The gruesome slayings of five students in three incidents as fall classes were starting two weeks ago unnerved this college town as never before. Gainesville, home to the University of Florida and Santa Fe Community College, liked to think of itself as a charmed and somehow invulnerable place.

Since the slayings, nearly 500 Florida students have withdrawn from the school. The student newspaper, The Alligator, is filled with ads offering Mace and courses in self-defense. College freshmen, away from home for the first time, have armed themselves with guns.

John Lombardi, the university president, said it is easy to understand why the stabbing deaths not only set off a near-panic in Gainesville but also struck a nerve across the nation.

"Gainesville is the quintessential college town," he said. "It's like Bloomington, Indiana. It's like Iowa City. This happened at a time when we're all busy sending our children off to college. It's every parent's nightmare."

Whether Gainesville, set among the marshlands of northern Florida, is the prototypical college town is debatable. But it does not really matter because the circumstances of the slayings were shocking enough to disturb most towns.

Four of the five were students at the university, and one attended Santa Fe. They were found semi-nude in their apartments between Aug. 26 and Aug. 28. Some were mutilated. All were slain by someone described by police as a "thrill seeker," who arranged each death scene as if, police said, a message was being left for them, although they would not elaborate.

As members of the National Guard prepared to comb swamps and woods around Gainesville this week for further clues, Lombardi sat in his office and pondered the contrasts between the serenity of his campus and the savagery of the crimes.

"There's no way to know when you'll be standing on the subway platform when somebody decides to start shooting," he said. "No way to know that you'll be standing in line at McDonald's when somebody goes crazy, and yet we all invent explanations that help reassure us that we're not susceptible to those kinds of violences. When it happens in a place like this, it's hard to invent the explanation. It makes us all very vulnerable all at once."

Lombardi, who sent his daughter and son to college this month in Indiana and Michigan, said students' parents had generated the rush to acquire firearms. "Parents' reaction to the risk of their children tends to be dramatic and profound," he said.

Lombardi said he is working to persuade students to abandon weapons as a form of self-defense. Progress on the case helps his effort, but the investigation has moved slowly, and uneasiness lingers.

Early on, police listed eight suspects, and they have zeroed in on Edward L. Humphrey, a troubled 18-year-old University of Florida freshman with a history of mental problems. Humphrey lived near one of the apartment complexes where the killer struck, and police have searched his car, a 1978 Cadillac, his apartment and his home in Brevard County.

Investigators said they have gathered 500 pieces of evidence and followed up on 3,600 tips. They are piecing together the backgrounds of the victims, four women and one man, to search for common threads.

"Did they go to the same hairdresser? Did they go to the same dentist? You have to really dissect their lives. When you take a look at serial killings, you don't see immediate arrests," said Lt. Spencer Mann, spokesman for the Alachua County Sheriff's Department.

Police have not been able to charge anyone in the crimes, and pressure to find the killer is intense. Humphrey is being held in Sharpes on $1 million bail on charges of assaulting his grandmother, Elna Hlavaty. She has said she has no interest in pursuing the charges, and she is not expected to testify against him. He also is charged by campus police with assaulting two male Florida students.

"This is not a typical homicide," Mann said. "This person was driven by certain desires, certain messages, a certain meaning in life that doesn't jibe with everybody else in society."

Police caution that Humphrey is only one of many suspects being pursued vigorously. But his photograph has been so widely published in Florida newspapers that it no longer needs an identifying caption. His lawyer, J.R. Russo, has sought unsuccessfully to have Humphrey's bail reduced.

In Gainesville, there is an attempt to return to normalcy. The Florida football team, Gainesville's most beloved institution, trounced Oklahoma State here last Saturday in the season opener, 50-7.

At the Purple Porpoise, a student tavern along University Avenue, business stood still after the slayings but is coming to life again.

"The game helped," said Mary McConnell, a graduate student who works as a waitress.

After the murders, she and three roommates installed a new deadbolt on the front door, 11 window locks and installed a rod to secure the sliding-glass door.

McConnell has her boyfriend pick her up when she leaves work at 2:30 a.m., and she and her roommates sleep with detached table legs next to their beds.

"The scariest part is going home," she said. "My dad said he was sending me some Mace."