WASHINGTON (AP) _ U.S. agents found it ``a little bit odd'' when they encountered Russian diplomat Stanislav Gusev near the State Department last summer. The chance sighting, officials said Thursday, led to the discovery of a Russian operation that included an ``extremely sophisticated'' spying device inside America's diplomatic headquarters.

It also triggered an expulsion order for Gusev and an intense in-house investigation to determine who was responsible for planting the device. Officials said hundreds of State Department employees were being interviewed.

The administration said the seriousness of the episode should not be understated.

``There are nations all around the world that are, for a variety of reasons, trying to gain access to information from us, and we just can't let our guard down,'' said Deputy Attorney General Eric Holder.

Neil Gallagher, assistant director of the FBI's National Security Division, said, ``I think this incident, by itself, sends a strong message that there is a very aggressive Russian intelligence presence operation inside the United States.''

The Cold War ended almost 10 years ago but strains in Washington's relationship with Moscow are sometimes reminiscent of that bleak era.

Last week, Russia ordered the expulsion of a U.S. diplomat in Moscow, Cheri Leberknight, after accusing her of attempting to obtain secret military information from a Russian citizen.

U.S. officials said there was no connection between the two expulsions. But Vladimir Lukin, a former Russian ambassador to Washington, said in Moscow the administration ``cooked up'' the charges against Gusev to retaliate.

Also last week, the Navy disclosed it had charged a petty officer with espionage for allegedly providing Russia with highly classified information about U.S. eavesdropping.

Among the divisive diplomatic and security issues between the two nations is the Clinton administration's proposal to amend the 1972 Anti-Ballistic Missile treaty to permit development of a U.S. missile defense system. The United States says it needs the system to protect against potential nuclear threats from Iraq and North Korea, but Russian military officials believe the real target is Russia.

As for Gusev, after his initial sighting, the FBI and State Department security agents discovered that he would turn up outside the department on roughly a weekly basis, leading to suspicions that he was receiving transmissions from a device planted inside the building, using equipment installed in his car.

Finding such a device was no small task, considering that the building, located a short distance from the Lincoln Memorial, is eight stories high and covers two square blocks. Thousands work there. Gallagher said a painstaking search exposed ``an extremely sophisticated device'' that had been ``professionally introduced'' into the 38-year old structure.

Dave Carpenter, the department's top diplomatic security official, said the device was not planted in the offices of Secretary of State Madeleine Albright or any other senior official. He declined to say where it was found, but other sources said the location was one of the department's many conference rooms.

After the device was found, word was passed to department officials not to discuss sensitive information in the room. The device was left in place so that the FBI and the department's security personnel could firmly establish a Russian link

Officials offered no hint as to what information may have been transmitted to Gusev's car before the discovery. Gallagher said there was no suggestion the Russian timed his arrivals to coincide with important meetings.

But his habits suggested to agents that he was no ordinary sightseer. As an example, Gallagher said Gusev would move his car several times during his visits, apparently in an attempt to find an ideal location to pick up conversations inside the building.

On Wednesday, Gusev showed up outside the department in late morning, about three hours before President Clinton was to hold a news conference in the building's main auditorium. Gusev was detained by agents shortly after 11:30 while sitting on a bench, and he quickly claimed diplomatic immunity. He was later informed that he would be given 10 days to leave the country.

Undersecretary of State Thomas Pickering summoned the Russian ambassador to the State Department to protest the eavesdropping. There was no comment Thursday from the Russian Foreign Ministry or the Russian Embassy here.