The FBI has launched a major investigation into allegations from a former KGB employee that the Soviet Union had "many hundreds of Americans" serving as spies during the latter days of the Cold War, according to a new book on the bureau.

Ronald Kessler, author of "The FBI: Inside the World's Most Powerful Law Enforcement Agency," said in an interview that former high-ranking U.S. government officials are among the suspects. At least one U.S. military employee stationed in Germany already has confessed after being confronted by the FBI, Kessler said.

A U.S. intelligence official confirmed that the FBI had received specific information that has led to a "significant" ongoing investigation into past KGB activities in the United States. But the official said that it would be speculative to say how many people might be implicated.

An FBI spokesman declined to comment about any specific investigation. But in a statement issued late yesterday, the FBI said, "Based on information that continues to be received in the aftermath of the Cold War, the FBI, pursuant to its foreign counterintelligence responsibilities, has opened a number of cases related to the activities of the former KGB and its successor agency. The process of thoroughly analyzing and evaluating that kind of information continues."

The prospect that the former Soviet intelligence agency was operating such a vast, high-level effort among Americans suggests a far more serious breach of national security than ever suspected. It also indicates that the bureau's counterintelligence division, which is being cut back in the aftermath of the Cold War, may find itself active for many years handling fallout of cases that may be developed from the investigation.

Kessler, a former Washington Post reporter who has written extensively on the U.S. intelligence community, was granted unusual access to FBI officials for his book, under an arrangement approved by then-Director William S. Sessions.

During the course of his research, Kessler uncovered information about alleged travel and other abuses by Sessions. Those allegations, outlined in a letter sent by Kessler to the FBI last year, were then passed along to the Justice Department's Office of Professional Responsibility, setting off the chain of events that led to Sessions's dismissal by President Clinton last month.

Kessler writes in his book that the information about the espionage came from a former KGB employee "who had had access to KGB files." According to the employee's account, "the KGB had had many hundreds of Americans and possibly more than a thousand spying for them in recent years," Kessler writes. "So specific was the information that the FBI was quickly able to establish the source's credibility."

The bureau has mobilized agents in most major cities to work on the case and recently called a secret meeting to plot strategy at its Quantico, Va., complex, according to Kessler's book.

The information about the KGB probe is one of several disclosures about bureau activities in the book. Kessler recounts the story of numerous recent FBI investigations from the perspective of bureau agents.

In one example, he says that top bureau officials may have misled the public about its internal deliberations leading up to the April 19 tear gas assault on the Branch Davidian compound in Waco, Tex. After the FBI action, cult members set a fire that leveled the compound, and 77 bodies were found in the rubble.

Sessions and other bureau officials had said at the time they felt it was unlikely that cult leader David Koresh would lead his followers in a mass suicide.

But according to Kessler's book, FBI agent Peter A. Smerick, when asked to profile Koresh, had written a memo "that the Davidians would indeed commit suicide if the FBI confronted them."

Smerick and Tony Daniels, assistant director in charge of the FBI's training division, confirmed this yesterday. But Daniels said while the prospect of mass suicide "was always in the back of everybody's mind," the consensus of FBI behavioral scientists and outside experts was that suicide was not likely.