A group of Americans is at work daily now sifting through the tons of rubble in the burned upper floors of the U.S. Embassy here, salvaging what they can and feeding the remains - papers and documents so badly scorched they are unusable - into something called the "disintegrator."

This powerful pulping machine can turn a coke bottle into glass dust instantly or reduce paper to a palmful of unreadable lint. Its shreddings are dumped down an empty elevator shaft into a waiting Soviet dump truck. With the atomized fluff in the back of the truck and short, sawed remains of rafters, posts and beams thrown from the roof by navy Seabees working to clear the charred wood and metal and raise a new roof.

When the truck is full, it pulls away from the embassy, reportedly for a trip of perhaps more than 50 miles outside the city, well beyond the limit Americans can travel without special permission. There, in a field and perhaps in a building as well. Soviet investigators are said to carefully sift through the broken timbers and the piles of fluff churned out by the "disintegrator," seeking clues to what the secret upper floors of the chancery housed.

Americans here are sure the search is unsuccessful.

The picture presented above, drawn from interviews with well-informed informed Western sources, cannot be confiremed in every detail. But several sources insist that it is accurate, although they decline to explain how they know what the Soviets are doing with U.S. rubble in a location far from here.

But what the sources say tells, as well as anything is recent days, much about the cat-and-mouse security contest that has been going on since the Aug. 26-27 fire at the U.S. Embassy on Tchaikovskova Street, a mile and a quarter from Red Squre. And it indicates the atmosphere of caution and guarded suspicion that prevades the American diplomatic community.

American sources conceded that the elaborate super-shredding of discarded documents may slow down the reconstruction of the gutted eighth floor offices of the chancery, which contained economic, cultural and press offices. "But this is the way it has to be," said one source. "It's all got to go through the disintegrator."

Equipment, such as communications items and related cipher equipment damaged in the fire, will be sent by scaled diplomatic pouch to Helsinki for shipments on to the United States, where the machinery either will be repaired or destroyed. "Ordinarily we might try to repair it ourselves, but there isn't time," said a source.

Despite evidence of what he called "minor looting," Ambassador Malcolm Toon has said he is confident no serious security breach occurred.

For the Soviets, the fire presented a unique opportunity to get inside a place they have said for years contains electronic monitoring equipment that eavesdrops on telephone and radio communications around the city. Some Soviet sources have cited this alleged activity by the embassy when queried about the microwave bombardment they have directed against the upper floors of the embassy, apparently since the early 1950s. Narrow aluminum mesh screens cover the windows now and sources say they stop nearly 90 per cent of the energy directed at the building's windows.

Some veteran observers here have suggested that the embassy's extreme security precautions are essentially futile because the Soviets, like the Americans, have advanced computers and electronic equipment to intercept virtually thousands of messages at once and quickly decipher them. The U.S. National Security Agency, for example, is believed routinely to have the capability to intercept telephone messages by picking up minute radiation emitted over phone wires from satellites listening from deep in space.

"All these things may be true of the Russians," one source said here, "but we must burn and shred things just the same. If this had happened to us in London, do you think we'd be any less careful? This is the only way to do things. I think it does some good."