A former Army sergeant arrested on espionage charges Thursday night had robbed a bank in Vermont before receiving a top secret Pentagon clearance and then joining a West Germany-based espionage ring that did "unprecedented" damage to U.S. security, an FBI counterintelligence expert testified yesterday.

FBI agent Joe Navarro said in a court hearing in Tampa, Fla., that Roderick James Ramsay, 28, admitted in "numerous" interviews prior to his arrest that he was a "close associate and confidant" of convicted spy Clyde Lee Conrad and had helped Conrad sell large quantities of classified documents to the Hungarian and Czechoslovakian intelligence services beginning in 1983.

These documents, including many involving NATO conventional and nuclear defense plans, later were passed by the Hungarians and Czechoslovakians to the Soviet KGB, Justice Department spokesman Doug Tillett said here. Navarro, who headed the investigation, said, "It's one of the most serious breaches ever -- it's unprecedented what went over to the other side."

The arrest of Ramsay on a downtown Tampa street Thursday night, occurring just a few days after Conrad was convicted of treason this week in a West German court, adds a new chapter to an espionage saga that federal officials now say appears to have been far more serious than when it was first disclosed in August 1988, when Conrad was arrested.

Ramsay did not enter a plea during yesterday's hearing before a U.S. magistrate, and was ordered detained without bail pending another hearing next Tuesday. Mark Pizzo, his court-appointed lawyer, did not return repeated telephone calls.

Navarro described Ramsay as a highly intelligent but erratic drifter, who spoke Japanese and German and "had the ability to recall minute details, facts, and figures, some from documents he hadn't seen in five or six years."

But at the same time, Ramsay apparently profited little from his alleged espionage activities. Since leaving the Army, he worked at odd jobs, including driving a taxicab, and most recently was living with his mother and working with her in a bookkeeping business for local law firms. In a financial disclosure form he filled out after his arrest, Ramsay listed his income as $1,000 a month and said he had no other assets.

By contrast, Navarro testified yesterday that Conrad, who was accused of leading the spy ring since 1975, received between $2.2 million and $5 million for his espionage activities.

Navarro testified that before joining the Army on Nov. 17, 1981, Ramsay had robbed a bank in Vermont and, while working as a security officer in a hospital, had attempted to break into a safe. No further details were provided. A federal prosecutor in Tampa later described the bank robbery as "unsolved" and said there was no indication that Ramsay was charged in the crime.

Ramsay received a top secret military security clearance and was assigned to work under Conrad as assistant classified documents custodian of the G-3 plans section of the Army's 8th Infantry Division in Bad Kreuznach, West Germany, about 50 miles east of Frankfurt.

As part of the Army's 5th Corps, the 8th Infantry Division has responsibility for defending central West Germany in the event of a Soviet Bloc attack.

According to Navarro's affidavit, Ramsay said that after being recruited by Conrad, he received $20,000 in exchange for passing along extensive classified material, including documents dealing with "the tactical use of nuclear weapons" by NATO forces and "General Defense Plans" for Western Europe. The defense plans, Army officials said, spell out where each military unit would be deployed during an attack and details what their missions would be.

Ramsay left the Army in November 1985 with a good conduct medal, an Army achievement medal, an active service ribbon, and an overseas service ribbon. Navarro said that Ramsay had later stored stolen top secret documents in his mother's house in Tampa and destroyed them after Conrad, who had left the Army by then, and seven others were arrested in Europe in August 1988.

The disclosure of his top secret clearance follows a series of highly publicized espionage cases, including the Walker scandal, that in recent years have prompted the Defense Department to sharply tighten its security clearance procedures. John Anthony Walker Jr., convicted of leading a family spy ring that sold Navy communications secrets to the Soviets, had received repeated clearances despite his conviction as a teenager in a string of burglaries.

Without a public record of criminal activity, there were no grounds to deny Ramsay a security clearance, an Army spokesman said.

But for all the severity of the potential damage to U.S. security from the case, officials acknowledged that the fallout may be muted by recent events in Eastern Europe.

A Hungarian government spokesman noted yesterday that, even before the election of a new democratic government in April that is pledged to negotiate the withdrawal of Soviet troops, the Hungarian Foreign Ministry in a statement earlier this year disavowed the use of the Conrad spy ring as an "erroneous policy."

"The previous government has already distanced itself from this case and indicated this is not the way we are going to proceed," said Gabor Szentivanyi, spokesman for the Hungarian Embassy here.

Staff writer Molly Moore contributed to this report.