Federal investigators are questioning State Department employees, cleaning workers and others who had access to a seventh-floor conference room where a listening device was planted inside a piece of molding, U.S. officials said yesterday.

The bugging of the State Department became public knowledge this week after the FBI detained Stanislav Borisovich Gusev, a Russian diplomat who was caught outside the building while operating the device, law enforcement officials said. Gusev, who has been in the United States since March, was turned over to the Russian Embassy after claiming diplomatic immunity.

The breach of security on the seventh floor of the State Department, where Secretary of State Madeleine K. Albright and other senior officials have offices, has left investigators puzzled. The FBI and State Department security service are seeking to determine how the bug was placed in a chair rail--a waist-high molding designed to protect a wall from chair marks--in the conference room of the department's Bureau of Oceans and International Environmental and Scientific Affairs.

While that room may seem an odd target for bugging, FBI officials said earlier this week that the device may have been put in a particular spot merely because lax security provided an opportunity. They also said the installation of the bug appeared to require advance knowledge of the size and shape of the room and could not have been accomplished in a single visit.

David Carpenter, the State Department's assistant secretary for diplomatic security, said hundreds of people who are known to have entered the conference room at various times are being interviewed, not necessarily because they are suspects, but because they may have information that could help investigators.

Carpenter, who declined to confirm where the bug was found, said some repairs have been made recently to the conference room where the listening device was discovered, including new ceiling tiles and a new door. The State Department recently beefed up security in the building, where visitors were allowed unescorted access to many areas until this fall.