The FBI will probe allegations of physical abuse of more than 20 residents by two former employees at a state home for the retarded in Fairfax County, the center's director said yesterday.

The FBI's investigation comes after a decision by Commonwealth's Attorney Robert F. Horan not to pursue prosecution. Horan said yesterday that the statute of limitations for simple assault, a misdemeanor, had run out and that "there was not enough evidence to prosecute."

David Lawson, director of the 285-bed Northern Virginia Training Center, said, "I'm delighted that there are continuing efforts to see what can be done about prosecution."

Lawson said he learned last week of the FBI's investigation, but declined to comment on specifics. "They are looking at the possibility of prosecuting under any federal statutes," Lawson said.

The FBI's field office in Alexandria did not answer a reporter's inquiries yesterday, and an FBI spokesman in Washington said the agency would not confirm the existence of an investigation.

Two employees of the center were fired in January 1987 after an undercover police investigation led to allegations of physical abuse, Lawson said at that time. An aide at the center had told an undercover state police officer that he and the two employees had, over several months, disciplined some of the retarded male residents at the center by punching or kicking them, state officials had said. The aide was transferred, Lawson had said.

The Northern Virginia Training Center, on 87 wooded acres near Braddock Road, is one of five residential centers for the retarded operated by the state.

In a letter Tuesday to Del. Robert E. Harris (R-Fairfax), Commissioner Howard M. Cullum of the Virginia Department of Mental Health, Mental Retardation and Substance Abuse Services, wrote:

" . . . Since no action was taken by the local authorities, the Federal Bureau of Investigation will apparently now go forward with their review . . . . This FBI investigation, under the direction of the U.S. Attorney's Office, was initiated by the Justice Department and could result in charges against former staff of conspiracy to violate the civil rights of certain residents of the Training Center."

The commissioner also wrote that he had been informed that an FBI investigator plans to visit the facility within the next several days to "interview a number of staff and to review certain records."

The unit where the alleged abuse occurred housed 28 males, ranging in age from 9 to older than 30. Twenty-six of the residents were thought to have been mistreated, the center director had said.