Fire inspectors and arson investigators huddled in small groups outside 2201 Massachusetts Ave. yesterday, frustrated in their attempts to enter the remains of the historic building and learn the cause of a spectacular three-alarm blaze that left the once-elegant mansion a charred ruin.

"We're not going to be able to get inside until the walls are shored up to make it as safe as possible," said Capt. Richard Clark. Examination of the debris will help solve certain "unanswered questions" about the cause of the fire, which has been classified as suspicious, he said.

Hundreds of people filed past on foot yesterday -- business executives on lunch breaks, parents with children in tow and neighbors lamenting the loss of what historians considered a cultural landmark and classic example of Queen Anne and Romanesque Revival architecture.

Many snapped pictures of the black rubble and sagging brick shell, about the only remains of the blaze that witnesses say swept through the four-story, $2 million mansion Wednesday evening.

Motorists detoured in a steady stream down Embassy Row to crane their necks and gawk at the crumbling facade of the Beaux-Arts mansion, located across 22nd Street NW from the luxurious Cosmos Club. Loose bricks, broken glass, a rusty fire escape and other charred debris cluttered the small front yard, where they fell when the third and fourth floor exterior walls collapsed during the fire.

Officials said the blaze apparently began in a rear room on the first floor about 6:30 p.m. The fire has been classified as suspicious, a fire inspector said, because of the intense speed with which flames devoured the house, which was vacant and had been undergoing renovation.

The Bureau of Alcohol, Tobacco and Firearms also has been called in to investigate the blaze.

Fire officials, who looked at the interior from a cherry-picker crane, said nothing remained of the mansion's distinctive features: mosiac floors, fluted Ionic columns, ornamental wood carvings and decorative friezes, many of which had nautical themes. The home was designed for a wealthy sea captain in 1901 by the associate architect of the Library of Congress, Paul J. Pelz.

Police and fire officials yesterday kept 22nd Street closed to traffic between Massachusetts Avenue and R Street, and also roped off the sidewalks surrounding the corner lot, afraid that the remaining walls may crash to the pavement.

The owner of the building, identified by fire officials as Bruce H. (Scott) Macleod, met with an architect with Richard Ridley & Associates yesterday to decide what parts of the building, if any, can be salvaged, according to a fire official.

"They want to save as much as they can," the official said. "They want to save the whole building, but especially the masonry. We don't have craftsmen who can do that kind of work nowadays. It has a lot of historical value."

Neither Macleod nor a spokesman for the architectural firm could be reached for comment yesterday.

According to Lusk's Reports, a directory of D.C. tax assessments, Macleod owns a number of other Northwest Washington homes and condominiums. He bought the mansion, also known as The Argyle Guest House, in December 1983 for $600,000, according to Lusk's.

Macleod, who is employed by the International Finance Corp. at The World Bank, was about 90 percent finished with renovating The Argyle from a 35-unit boarding house into nine luxury condominiums, officials said. They said that contractors were working on "framing out" the structure the day of the blaze and left about two hours before the fire was discovered.