Metro officials are investigating the theft of an undetermined amount of money by their employees from some of the more than 200 farecard vending machines located in the Metro subway stations, The Washington Post has learned.

"I really don't think it's a lot of money, I think it's very small," said Delmar Ison, Metro's secretary-treasurer.

The investigation began about two weeks ago, according to Angus MacLean, director of security for Metro. No one has been arrested but some people have been questioned, MacLean said.

"The problem, frankly, is the people with keys" to the machines, Ison said. He stressed that "we have a lot of attendants we think are honest people."

The vending machines, the 4-foot-by-8-foot monsters that freet Metro riders at the bottom of the escalator, are supposed to take money and dispense the magnetically encoded tickets that let passengers board the trains.

Three catergories of employees have keys that let them open the front of the machines: the station attendants, the repairmen and the revenue collectors. Only the revenue collectors, who are always accompanied by a security guard, have a second key that is necessary to remove the money boxes from the machines.

When the money boxes are removed, they lock automatically. Their contents are then transfered to revenue wagons through a series of automatic locks. The revenue wagons are opened in a vault underneath Metro headquarters.Theoretically, money cannot be stolen without breaking into the boxes or the revenue wagons.

When a patron buys a farecard, the money he deposits in the machine is supposed to go directly to the money box and thus be beyond the reach of even those who have keys.

However, money, especially dollar bills, sometimes hangs up in the machines and can be retrieved by an attendant or repairman with a key. Many attendants regularly do retrieve money for irate customers who deposit bills and get an "out of order" sign instead of a farecard.

"No one has gotten into the vaults (money boxes)," Ison said. "Some have found a way to get to a limited amount of money." He refused to specify what that way would be or to give any estimate of how much money is missing beyond the words "small amount."

MacLean said that the incidents appeared to be limited to a few locations in Metro's 24-station network. At any one time, more than $600,000 is tied up in change dispensers for Metro's farecard machines.

"When we get our equipment working with 100 per cent accuracy," Ison said, "we will have audit trails that can track this kind of thing machine by machine. But we don't have that yet."

The farecard equipment - including entrance and exit gates - is supposed to tell Metro exactly how many people ride, where they get on and off, what they pay, ect. But that equipment has been operating with about a 15 per cent error rate, according to weekly Metro statistics on ridership.