An internal Fairfax County police investigation has cleared a police officer who mistakenly shot a janitor during a stakeout of an often robbed school, Commonwealth's Attorney Robert F. Horan Jr. said yesterday.

Horan said the investigation found "no wrongdoing" by officer David Lubas, who shot and killed custodian John Jackson Dec. 29 in the kitchen of the private Talent House School, 9211 Arlington Blvd.

Lubas shot Jackson three times when he entered the school with his 22 caliber gun, apparently to begin his own stakeout.Jackson was unaware that two plain clothes police officers were also staking out the school.

"It's a tragic incident," Horan said. "There is no reason to suspect that Jackson was coming in there to burglarize the place. He was coming there to stake the place out like the police [were doing.]"

Eldon Merritt, owner of the school, had offered a $500 reward to his employes for help in apprehending the persons responsible for a series of burglaries at the school.

Jackson, 33, of Manassas, had returned to the school about 1:30 a.m. on Dec. 29. But, unknown to him, two Fairfax County police officers, Lubas and Nancy Lutz, were inside the school on a stakeout.

"Normally on a stakeout, you let as few people know about it as possible," Horan said. "There is a question of whether or why Mr. Jackson wasn't told about it. Certainly, he should have been notified."

David Feldman, an attorney who is investigating the death for Jackson's family, said yesterday that Jackson had staked out the school premises the three nights prior to the shooting. "It's absolutely obvious that something went wrong," Feldman said.

Horan said Lubas, a three-year police veteran who had been placed on administrative leave pending the completion of the police investigation, returned to work last week.