An internal police investigation has cleared the Prince George's County K-9 officer who shot 18-year-old Michael P. Sluby to death Friday in a former Mount Rainier school building, a police commander told a press conference yesterday.

"After a thorough review" of the case, Maj. James Ross said, the chief of police found that "Officer Joseph Wing acted in self-defense." Ross followed his remarks with the first official account of how the shooting took place, a version later disputed in part by the families of Sluby and Candace Jean Craig, Sluby's girlfriend who accompanied him to the school and was charged later with breaking and entering.

Craig told police that she and Sluby found a tape recorder in the school and decided to take it, Ross said. She told investigators that minutes later she and Sluby heard the dog and assumed that either "a watchman or police officer" was in the building, Ross said.

Ross said the K-9 search was conducted according to routine procedures. Five officers responded to the silent alarm tripped at 12:33 a.m. when Sluby and Craig slipped through a broken window on the ground floor of the school, which was closed three years ago and was to be the site of a Halloween party.

The officers stationed themselves around the building and Wing, who was in uniform, announced over a loudspeaker, " 'County Police K-9 -- come out or I'll turn the dog loose,' " Ross said. Hearing no response, Wing unleashed the dog, Rebel, and it began to track the intruders.

"The search ended," Ross' statement said, "when Rebel and Officer Wing entered a dark classroom on the ground level across the hall from the gymnasium. At this point, Officer Wing's revolver was holstered and he was carrying his flashlight. As the officer entered the room, his dog immediately attacked Mr. Sluby, who was standing against the wall. At this point, Officer Wing observed Michael Sluby stab his dog . . . . After stabbing the dog, Mr. Sluby advanced toward Officer Wing with the knife still in his hand. Officer Wing drew his revolver from his holster and fired two shots, striking Mr. Sluby in the arm and chest."

Police released a photograph of the knife allegedly used by Sluby to stab the dog twice, once in the shoulder and once in the back. The photograph indicated the folding-type knife had a blade 4 1/8 inches long. The size of the knife has been in dispute, with police previously stating it was "rather long" and friends and family of Sluby contending that Sluby normally carried a knife whose blade was about 2 1/2 inches in length. Police said they also recovered a wire cutter from Craig's coat.

Officer Wing was about 10 feet from Sluby when he fired and did not have time to say anything to Sluby before shooting him, Ross said.

"It was a very quick sequence," Ross said. Craig had slipped out of the room and did not witness the stabbing and shooting, he added.

Barbara Sluby, the slain youth's mother, said she was "not surprised" that the police investigation cleared Wing, who was taken off administrative leave yesterday. "It sounds strange to me that he Wing didn't have time to identify himself," Sluby said. "It seems to me that if he had control of his dog, there would have been time for quite a few things."

Robert Craig, father of Candace Jean Craig, disputed the statement that his daughter admitted she and Sluby planned to take the recorder.

"They weren't there to steal anything out of that place," Craig said. " . . . I don't know about any statement. She might have said that under duress and in complete shock. The kid is still not in good shape. To see someone get wasted like that."