A former KGB colonel who was a top double agent for British intelligence has said he cannot explain a major mystery that has developed over whether he was first betrayed to Moscow in 1985 by confessed U.S. spy Aldrich H. Ames or by another unidentified source.

But Oleg Gordievsky, who survived his exposure thanks to a dramatic escape from the Soviet Union nine years ago, said in a telephone interview from Britain last week that he continues to believe it was Ames who turned him in.

After six weeks of questioning Ames in three-day-a-week sessions, the FBI and CIA remain baffled about whether Ames or someone else first warned the Soviets about Gordievsky, according to sources familiar with the issue.

The question is critical. If Ames did not pass Moscow the initial information about Gordievsky, then there may have been another informer inside the CIA or elsewhere helping the Russians.

The confusion arises because Ames cannot remember exactly when he first gave the Soviets information related to Gordievsky, the best Soviet spy ever recruited by British intelligence. An FBI reconstruction of events puts the date on or about June 13, 1985.

That, however, is weeks after May 17, 1985, the day Gordievsky says the KGB suddenly ordered him to return to Moscow from London. Ten days later Gordievsky was accused by his superiors of being a British spy.

If Gordievsky's dates and Ames's story are correct, either the FBI's reconstruction of events in June is wrong or someone other than Ames initially made the KGB suspicious of Gordievsky.

Gordievsky said in the interview that he cannot explain the June date set by the FBI. "If Ames confirmed me {to the KGB} in June, they would have arrested me," he said. "In June, nothing else happened to me except surveillance and listening."

Ames, who is incarcerated in Alexandria, has been interrogated since he pleaded guilty April 28 to espionage charges. Interrogators have begun to question his story about passing on information about Gordievsky to the Soviets. They also are trying to find out from Ames and others whether there might have been another source who knew enough to lead the KGB to Gordievsky.

In recent interviews, Ames and Gordievsky, both professional intelligence operatives who betrayed their respective countries, provided some details about the incident but could not clear up the matter.

Sometime in mid-1985, Ames delivered to a Soviet Embassy contact a package containing his list of all the U.S. and allied-paid Soviet agents he knew.

Although he did not know Gordievsky's name, Ames and several CIA colleagues had figured out that someone high up in the KGB in London was delivering highly sensitive material that British intelligence was often hand-delivering to the CIA for President Ronald Reagan. Ames also knew the British source had a relationship with Danish intelligence.

Ames gave the Soviets enough identification, one source said, that "would inevitably have led" the KGB to Gordievsky. Ames has said he remembers turning over the package "several months" after handing over an initial list of two or three Soviet names in April 1985.

At least 10 persons identified by Ames either were tried and killed or disappeared. Ten days after he returned to Moscow, Gordievsky was drugged and questioned by KGB superiors about being a British spy. After his repeated denials, Gordievsky was turned loose by the KGB, but kept under surveillance. On July 19, while out jogging with a KGB guard close behind, British intelligence agents seized him and put him on his way to an amazing escape.

Gordievsky said that British intelligence had established that the Soviets first focused on the possibility that he was a spy "between April 20 and May 10, 1985." During his May 27 questioning in Moscow, he said "one of my important {KGB} bosses" said they had learned about him from "a peculiar source, an unexpected source."

He said only an American source and one in the CIA would fit that description. Gordievsky also said he was told by his boss that they were looking for a KGB mole who had been "recruited by the British and had a Danish background."

Gordievsky said he was "saved" because the KGB did not have his name. "They couldn't arrest a KGB colonel without having a name" or some other hard evidence, he said.

Despite the fact that one turned in the other, Gordievsky and Ames voiced mutual respect. Each spoke glowingly of the information that the other had delivered. Gordievsky said "the significance of Ames was huge." Ames said everyone had admired the quality and quantity of what Gordievsky delivered. He said KGB agents "all over Western Europe" were slowly rolled up over the years thanks to Gordievsky's information.

They did differ on one basic point: Gordievsky said he acted out of despair for his country's direction and the corruption of its leadership. Ames said his main motivation was the several million dollars he received and was promised.