The hand-painted poster outside Mount Rainier Junior High School, the "E" made from a witch's peaked hat and pointed chin, the "T" a hatchet dripping blood, serves an ominous warning to passers-by: "Haunted School," the sign cautions. "Enter At Your Own RISK."

The poster, advertising a community group's Halloween decorations inside, did not scare away Michael Presley Sluby and his girlfriend, Candace Jean Craig. On their way from dropping off a friend early last Friday morning, according to Sluby's family, the pair of 18-year-olds spied a back window open at the dilapidated building, which has been closed for several years, and decided to investigate.

A short while later, after tripping a silent alarm to a local police station, Sluby was dead, shot twice in the chest by a Prince George's County policeman after fatally stabbing the officer's K-9 dog, an 80-pound, 3 1/2-year-old German shepherd named Rebel. Police said the officer was defending himself from an intruder who came at him in the dark, brandishing a knife.

Yesterday, as friends streamed through their house offering condolences and bringing food, the Slubys and others said they suspected the shooting was a tragic mistake, a Halloween prank gone horribly awry.

"He and his girlfriend, they were really up on Halloween. They had gotten all these masks, wanted to go to Georgetown . . . . I'm sure they went in to see how it was set up," Sluby's mother, Barbara, said as she sat in the living room of the family's modest home in a working-class neighborhood in far Northeast.

"With an open window," she said, "there was an invitation."

"He was never violent," said Sluby's father, Paul, who retired in 1980 from the D.C. police force after a career of nearly 20 years. "I have to seriously doubt that he would have attacked a police officer or threatened him in any way," he said of Michael, the youngest of four children. An older brother, Tom, a basketball star at Gonzaga High School and Notre Dame, joined the Dallas Mavericks this year and suited up for his first professional game yesterday.

Prince George's County police, who are conducting a routine investigation of the shooting by 12-year veteran Joseph Wing, 34, offered few details about the incident but said they believed Wing was acting within approved guidelines.

"Preliminary indications are that the officer did everything properly," said Col. Elmer Tippett, the second-highest ranking officer in the department, who was attending Rebel's funeral yesterday.

"I expect to be cleared by the internal investigation this week," Wing was overheard telling Tippett. "It's about as clear as you can get."

Police spokesman Bruce Gentile said that he did not know whether Wing identified himself as a police officer or ordered Sluby to drop the knife, but he said that Sluby and Craig "knew police were in the building." He said the weapon was "a rather long" knife.

"I don't know if he Wing had the opportunity" to identify himself, Gentile said. "It happened so quickly in a darkened room." Rebel had not attacked Sluby, according to Gentile, when the youth "stabbed the dog several times and turned and went toward the officer. He was advancing toward the officer, knife in hand."

But Sluby's family and friends say that story doesn't fit into their image of the University of the District of Columbia freshman as a gentle person who had never been in trouble. He was an animal lover, his mother said, apt to turn up with a wounded bird or baby squirrel, and -- much to her consternation -- constantly sneaking Sandy, his half-Labrador, half-German shepherd mutt, into his bedroom at night.

The knife Sluby was known to carry often, said his best friend of more than 10 years, had about a 2 1/2-inch blade and was used to carve clay figures for making animated cartoons in the style of Mr. Bill on TV's "Saturday Night Live." "Mom used to get mad when we used the butter knives or whatever so I think that's when he picked it the knife up," said Donald Reeping, who sat with his family watching a videotape the friends had made last month. The tape showed Reeping, two sisters, and Sluby giggling as they gyrated and lip-synched to the strains of rock star Prince.

Craig, Sluby's girlfriend of about six months, who was charged with breaking and entering in connection with the incident, was the valedictorian at All Saints High School in Northeast, from which the two graduated in June, according to friends.

"They were just regular nice kids," said Donna Young, who was a year ahead of Sluby and Craig at All Saints, formerly St. Anthony's High School. "They didn't do drugs, they didn't steal. They're not the kind of people you'd see walking down the street and lock your door."

"Michael is not just another statistic in Prince George's County," insisted Donald Reeping's mother, Theresa. "He's somebody special, not a hoodlum."

Craig, a freshman at the University of Maryland who was released on her own recognizance after being arrested Friday morning, declined to discuss the incident yesterday. But the Slubys said they had talked to Craig about the events.

Police said that the pair had moved an open reel tape recorder belonging to the school to a stairwell. Barbara Sluby said that Craig told her that the pair "had picked up the tape recorder to look at it and had moved it in doing so."

"Knowing Michael, he may have thought about recording some weird sounds to show it was haunted," she suggested. "Michael is that kind of a practical joker."