There had been a series of burglaries at the secluded, red-brick Talent House Private School off Arlington Boulevard, troublesome enough for owner Eldon Merritt to offer a $500 reward to his employes for help in stopping the crimes.

In the early hours of yesterday morning, part-time school custodian John Jackson, 33, of Manassas, armed with a 22 caliber revolver, returned to the darkened school building, drove up its long, tree-lined driveway and let himself through the kitchen door.

According to accounts pieced together yesterday, Jackson, a trusted worker at the school where he had been employed for six or seven years, believed the single-story structure was empty. It was a fatal mistake.

Waiting quietly inside, unknown to Jackson -- and to do the school's owner -- were two Fairfax County police officers staking out the premises.

Police said yesterday that the two, Offices David Lubas and Nancy Lutz, both in plain clothes, confronted Jackson and ordered him to put down his weapon. Jackson raised his hand, police said, Lubas fired three times, striking Jackson twice in the torso and once in the hand.

Jackson, who did not return the fire, was taken to Fairfax Hospital where he died at 3:37 a.m.

Lubas, a three-year veteran of the county police force, was placed on administrative leave yesterday pending the completion of an internal police investigation of the incident.

Merritt said yesterday he was shocked by Jackson's death. "I thought he was a terrific man. I thought the world of John. He would never take anything. I trusted him."

David Feldman, an attorney and friend of the Jackson family who said he had been asked to look into the death, said. "I just can't understand how the man got killed. I don't know whether it was somebody got trigger happy or what. It's one damn terrible mistake."

According to police and Merritt, the school, located at 9211 Arlington Blvd., had been burglarized five times between Oct. 20 and Nov. 5. Television sets, money and food were taken in the burglaries.

Officers from the Mason District station had staked out the school on 20 nights since Nov. 5, police said.

"I don't know why it happened,"

Merritt said, however, that he was unaware of the stakeouts. He said police apparently were given a key to the school by an assistant director on the staff. Merritt said Jackson was unaware of the stakeout.

said Merritt, who closed the school yesterday because of the shoting, "A good man's life was lost."

According to the police accourt, Lubas and Lutz arrived at Talent House after midnight. Officers on the stakeout generally went to the school after the cleaning crew had left, police said, and stayed until about 5 a.m.

Jackson, who also held a fulltime job as a laborer for Vepco, had cleaned the school, with this wife's help, on Thursday night. Then he drove his wife to Manassas and left her with the couple's 2-year-old daughter before returning to Fairfax.

Jackson arrived at the school about 1:30 a.m. He let himself in the back door with his key and walked in the back the ktchen. Lubas and Lutz stood silently in the main office where they could watch both the front and back entrances.

Police said Lubas then appeared at the kitchen doorway, indentified himself as a police officer and ordered Jackson to drop his gun. When Lubas fired. Police did not say whether the rooms' lights were switched on.

Deputy Police Chief Kenneth R. Wilson yesterday declined to comment on the case.