The Justice Department has begun an investigation into allegations that some employes in the U.S. Forestry Service's Rosslyn office may have submitted false overtime payments that cost taxpayers between $50,000 and $100,000, forestry service officials said yesterday.

Five employes in the office have left the service, part of the Agriculture Department, according an official in the service's personnel office.

Time cards for the Rosslyn employees have been taken by investigators from the U.S. attorney's office in Alexandria, according to Tim Walker, an employe relations specialist for the service.

"It was not a routine check," Walker said of the investigation. "Someone figured out how to milk the system."

William B. Cummings, U.S. attorney in Alexandria, refused to comment on the investigation against the employes, citing "possible grand jury investigation."

"It's a serious situation," said Walker . . . It's a real touchy thing . . . "

According to Walker, the Forestry Service has more than 800 employes who work in two Rosslyn locations. Time and attendance cards ("the t and a" worksheets) are filled out by timekeepers there who are assigned the extra duty of keeping track of who works what hours.

"I suppose it would take a timekeeper 10 minutes a day to fill out the "t and a" sheets," Walker said. Timekeepers receive reports of hours worked information from individual employes, he said.

"There are some checks and balances in the system," said Walker, "but not a whole lot. They were relying on people's honesty and integrity. Because of a few people who weren't honest, controls have had to be set up."

Walker declined to comment on what those controls were, but said they were a direct result of the recent investigation, which he described as "exhaustive."