West German investigators said today they are searching for a third suspect in an apparent Soviet Bloc espionage ring that came to light following the disappearance of two secretaries who are believed to have worked here as East German agents.

The federal prosecutor's office announced today that it is looking for a messenger from an Army administrative office in Bonn who vanished last weekend. The missing man, identified by security officials only as Lorenz B., is said to be a close friend of Ursula Richter, 52, one of the secretaries who police believe was infiltrated into the country by East German intelligence two decades ago.

In his previous employment with an elevator firm, the messenger helped install air-conditioning equipment in a massive bunker built under the Eifel Mountains. The bunker was to be used by the Bonn government as a command center in time of war. His work there would have given him full knowledge of the layout and life-support systems of the underground complex, security officials said.

The missing man later worked as a maintenance engineer in several government buildings, including one that housed the offices of members of parliament, according to a spokesman for the elevator firm Flohr-Otis, where Lorenz B. worked.

Security investigators, however, said they are even more concerned about the nature of the espionage work believed to have been carried out by the two missing secretaries, who now show signs of having worked here as important and well-trained East German spies for many years.

Police said evidence had been found at Richter's apartment indicating that she served as a control officer in charge of a network of secret agents operating in Bonn. She disappeared last Friday, a week after authorities reported that Sonja Lueneburg, 61, the personal secretary for the past 12 years of Economics Minister Martin Bangemann, was suspected of espionage and probably had fled to East Berlin.

Police subsequently have learned that both women lived under false identities after entering West Germany through third countries, a technique used by East German intelligence to cover suspicion about the backgrounds of their agents.

Richter, who had been under surveillance for some time as a possible spy, was a secretary in the accounts department at the League of Expellees, a lobbying group representing Germans driven out of Eastern European territories after World War II. Such a position would fit the kind of unobtrusive employment, with little or no security risks, used by East German control officers who manage several data-gathering spies.

Richter moved from Montreal to Bonn in the 1960's with the identification papers of another woman. Lueneburg entered West Germany about the same time through Colmar, France, with the identity of a West Berlin hairdresser who had settled in East Germany, police officials said.

If it is determined that Lueneburg was linked to East Germany espionage operations, the case could emerge as the most politically volatile one since Guenter Guillaume, a close aide to former chancellor Willy Brandt, was exposed as a communist spy in 1974. The scandal forced Brandt to resign from office.

While Lueneburg did not have top security clearance, her close working relationship with Bangemann enabled her to glean extraordinary insights into the government and political party structure. She was considered an intimate family friend, and Bangemann is said to have trusted her completely.

Investigators have interviewed the minister intensively about his dealings with Lueneburg. As head of the Free Democratic Party, which has participated in governing coalitions since 1969, Bangemann attends meetings of the Federal Security Council, which gives him access to the country's most highly classified information.

Bangemann, who broke off a trip to the Far East when he learned that his secretary had vanished, is said to be immensely distraught by the case and its implications for his career and the political fortunes of his party, according to well-informed sources.

The latest spy scandal demonstrates again the ease with which approximately 3,000 East German agents are said to have penetrated West Germany's institutions. Besides the absence of any language or cultural barriers, East Germany is known to exploit Bonn's willingness to give all Germans a passport by infiltrating agents within groups of political refugees who come to the West.

Moreover, the East German secret service is said to have enormous success in recruiting lonely West German women who work as secretaries in the Bonn government to perform espionage tasks.

Since 1975, 10 secretaries have been tried and convicted of spying for East Germany during the course of their work at Bonn's Defense and Foreign ministries as well as the Chancellery. Agents also have been discovered working as secretaries in key political party posts.