A 20-by-30-foot section of ceiling plaster fell to the floor of one entrance to the Farragut North subway station sometime yesterday morning while the station was closed. No one was injured.

Metro construction officials said yesterday that a full investigation was under way to determine why the ceiling had fallen. The station entrance, at Connecticut Avenue and K Street NW, will remain closed at least through this morning's rush hour, officials said.

The type of ceiling that fell is standard construction throughout the Metro system, although the material in which it is anchored varies from point to point.

Gen. John S. Egbert, Metro's new assistant general manager for design and construction, set up a specific program yesterday afternoon to determine exactly where else such types of ceilings are located and a schedule for detailed structural inspection.

However, visual inspections for cracks and sagging were completed at several stations yesterday, including the other two entrances to Farragut North and all the entrances to Metro Center. No imperfections were found. Those two stations were finished by the same contractor, Morrison-Knudsen Co. Inc., of Boise, Idaho. Inspections for structural integrity will follow, Ebgert said.

"The sequence in which we look at the rest of the stations will be based on who did the work," Ebgert said. "We are putting together a matrix right now of where this type of situation exists." Some of the suspended ceilings are hung from concrete, as is the case at Farragut North, others from steel, Ebgert said.

The fallen ceiling is located at the bottom of the bank of three escalators. It was discovered early yesterday morning by the station attendant who was opening the mezzanine for Metro's 6 a.m. start.

The false ceiling was suspended about two feet from solid concrete. Anchors forced into the concrete were supposed to hold the suspension wires. The wires hold a wire mesh lath. Plaster was applied to the lath. Inspectors were checking to determine whether the wires pulled loose from the anchors or the anchors pulled loose from the concrete. There was no evidence of water damage.

The warranty on the construction work apparently expired two weeks ago, Metro officials said. However, there is a three-year warranty on "latent defects," and, Ebgert said, "we're certainly going to get into that."

The Farragut North ceiling was part of $5.4 million in finish work performed by Morrison-Knudsen, one of the nation's largest construction firms. Morrison-Knudsen has been the prime contractor on a total of $43.9 million in Metro work alone.

Charles Hunt, contract administrator for Morrison-Knudsen here, said "I can't say anything about this because I don't know the facts."

The station work was designed and inspected by Rummel Klepper & Khal, a Baltimore firm, for $1.05 million. Emil Kordish, a partner in the firm who supervised the inspection, said, "I haven't heard a thing about it. It shouldn't occur unless something abnormal happened."

Metro riders departing at Farragut North could not reach the mezzanine where the ceiling fell because both escalators were turned in the down direction. Occasionally, but not always, announcements would be made that the entrance was closed as trains disgorged passengers.

"The K Street entrance is closed due to a mechanical problem," the announcement said.