Investigators searching for the cause of a suspicious fire at the Northwest Washington headquarters of the American-Arab Anti-Discrimination Committee believe the offices below the organization's were ransacked beforehand in a search for confidential documents belonging to the group, an official familiar with the investigation said yesterday.

The two-alarm blaze, which occurred Friday night, was still under investigation and has not been listed as arson. But the source said investigators found open file cabinets and drawers in the second floor offices of a public relations firm, with "stuff thrown all over the place." The ADC offices are on the fourth and fifth floors.

In addition, the source said, the first firefighters to arrive at the scene of the fire "thought they smelled an accelerant."

The 8 p.m. fire at 1731 Connecticut Ave. NW caused about $500,000 in damage to offices in that building and a grocery store at 1729 Connecticut Ave., D.C. Fire Department officials said.

Though most of the damage was confined to offices above the grocery store, those offices carry the 1731 address because that is the only access to them.

This latest suspected attack on the ADC drew denunciations and calls for police protection by Arab American groups.

"This is the third serious attempt to stifle the political dissent that comes from the American-Arab Anti-Discrimination Committee," ADC's national chairman, former U.S. senator James G. Abourezk (D-S.D.), said at a news conference yesterday outside ADC's charred offices.

"I just looked at the building, and it appears without other evidence that someone threw . . . an incendiary device," he said.

On Aug. 16, a police officer was injured when a bomb exploded outside the ADC's office in Boston, and on Oct. 11, a bomb planted at the ADC's Santa Ana, Calif., office killed Alex Odeh, 41, the organization's West Coast regional director.

A D.C. Fire Department source, who declined to be identified, said that there was no evidence to support Abourezk's contention that a firebomb was thrown at the building.

"I think that had there been an explosion, with all the people on Connecticut Avenue they would have heard it," the source said.

ADC's deputy executive director, Barbara Shahin, said she was working alone in the organization's fourth floor offices Friday night when she heard a "loud thud" about 30 seconds before the building's fire alarm began ringing. By the time she went downstairs a few minutes later to investigate the noise, she said, the front of the building was engulfed in flames.

Fire department officials said the blaze raced up the front of the building and spread through the third-floor offices of a security firm and then to the fourth-floor offices of the ADC. It took about 80 firefighters more than 45 minutes to bring the blaze under control.

"This young lady Shahin was lucky she got out of the fourth floor with the speed that fire spread, otherwise they the ADC would have had another death," said fire department spokesman Ray Alfred.

A source familiar with the investigation said the blaze started in the second-floor offices of Susan Davis and Associates, a public relations firm, and that files there were ransacked as the intruders apparently searched in the wrong office for confidential ADC documents. The search appeared to have been called off when a person or persons inside the offices discovered that someone else was in the building's upper floors, the source said.

He said that a laboratory analysis will be made of burned debris from the fire, which also is being investigated by arson agents from the Washington field office of the Treasury Department's Bureau of Alcohol, Tobacco and Firearms.

Abourezk said yesterday that the ADC's nine offices nationwide have received constant threats and harassment since the organization was started in 1980 to counter what it sees as discrimination and stereotyping of Arabs in the Middle East and Arab Americans in the United States.

The Washington office and its 25 employes will probably have to relocate as a result of fire damage, he said.

James Zogby, cofounder of the ADC who is now director of the Arab American Institute in Northwest, called on law enforcement agencies to provide additional security for Arab American leaders and their organizations, saying that there is widespread fear in the community about "who will be next and what organization will be next."

In a statement last night, spokesmen for the New Jewish Agenda condemned attacks on the ADC.

"As Jews working to promote the inclusion of all peoples into discourse favoring mutual respect and peaceful political processes, we applaud the work of the ADC and urge it to resume its operations as quickly as possible."