A federal transportation safety investigator said yesterday he suspects that an Eastern Airlines jet that made an emergency landing at Dulles International Airport Friday night had malfunctioning landing gear but added that investigators have not confirmed that theory.

The Miami-originated flight carrying 140 persons headed for National Airport made a wheels-up landing at Dulles at 6:51 p.m. Friday after the pilot and crew tried unsuccessfully to lower the wheels as they approached National. No injuries were reported.

Jack Young, a National Transportation Safety Board investigator heading the probe of the incident, said yesterday that he suspected it was "probably some malfunctioning in the gear system, but we haven't identified it yet."

Young said that during a preliminary check of the Boeing 727 yesterday, officials encountered difficulty getting the right main gear locked and had to force it down after the plane had been jacked up and towed from the runway.

Robert C. Welch, the plane's pilot, reached in Orlando yesterday, declined to discuss the incident, citing Eastern and NTSB policy discouraging comments until the NTSB inquiry is completed.

An Eastern spokesman said Friday night that the crew received a "gear unsafe" warning light and decided to abort a landing at National, which has shorter runways than Dulles.

An NTSB spokesman said it could be weeks before a preliminary finding is made and months before the cause of the incident is determined. The spokesman said the agency will conduct a review of maintenance records for the aircraft.

Eastern is already under investigation by the Federal Aviation Administration as a result of reports that the airline defers maintenance on many planes. An Eastern spokesman said Friday that there was "nothing wrong with deferred maintenance" itself, adding that any connection between deferred maintenance and the emergency landing could not be determined until the incident's cause is found.