A spectacular three-alarm fire destroyed an elegant $2 million Beaux-Arts mansion on Embassy Row last evening and damaged several nearby buildings, including the Embassy of Luxembourg.

D.C. fire investigator Joseph Fiore said the blaze, which erupted about 6:30 p.m., apparently started in a room at the rear of the first floor of the mansion and was "suspicious in origin."

"There was no rhyme or reason for that fire to spread that rapidly," Fiore said. He said the mansion was in the last stages of a renovation project to convert it into nine condominiums and was not occupied when the blaze broke out.

The fire swept within minutes through the 83-year-old architectural landmark, designed by the associate architect of the Library of Congress, and was burning on all four floors and through the roof when firefighters arrived.

Late commuter traffic in the area just west of Dupont Circle ground to a standstill as a 16-block area was closed around the burning building at 2201 Massachusetts Ave. NW.

Hundreds of office workers, executives and nearby residents -- joined by entertainer Pearl Bailey -- gathered along Massachusetts Avenue and nearby streets to gawk as flames flared 40 feet above the roof and black smoke billowed over the neighborhood.

Within two hours, the classic example of Queen Anne and Romanesque Revival architecture had been transformed into a hulking ruin, eerily backlit by powerful floodlights.

The blaze spewed fiery cinders over the area of 22nd Street and Massachusetts Avenue, but the only major damage to other structures was reported in an adjoining town house at 2203 Massachusetts Ave., which shared a common wall and chimney with the mansion.

Fiore credited quick action by 2nd Battalion Chief Joseph Dorack with saving the town house next door, where damage was estimated at $85,000.

Fire officials said flames were flaring from every window of the mansion and spiraling from its peaked and turreted roof when firefighters arrived.

By 8:30 p.m., the roof and entire interior of the four-story brick structure -- built for a wealthy Navy veteran in 1901 -- collapsed into the basement. Then some parts of the exterior walls collapsed to the second-floor level, where bricks had been "bowed out as if they had been pushed out," fire officials said.

A fire inspector said the intensity of the fire and the danger posed by falling debris made it impossible for the 150 firefighters at the scene to do little more than train hoses on the mansion and other buildings in an effort to keep the fire from spreading.

Sixth Battalion Fire Chief B.C. Johnson said that all the firefighters were able to do was "just hit it with heavy streams from a distance and just flood it."

As the blaze continued to roar out of control, firefighters were dispatched to the adjoining town house to search for hot spots in the common wall, and also to the roof of the Embassy of Luxembourg across the street, onto which flaming cinders were drifting.

Ambassador Paul Peters said he was in the embassy when he spotted the fire breaking through an upstairs window in the mansion.

"It was a small fire, and then it blew up," Peters said, and spewed smoldering cinders onto the embassy grounds. He said cinders also badly scorched the red, white and blue Luxembourg flag that hung above the entrance.

Another neighborhood resident, Mark Trautwein, said he had noticed nothing amiss when he walked past the house shortly before 6:30 p.m. on an errand. When he returned minutes later, "It was like a Roman candle . . . . You could feel the heat a block away."

More than a hundred spectators remained at the fire scene late last night as firefighters poured thousands of gallons of water into the cracked and crumbling shell of the building in an effort to douse hot spots still burning in the basement.

Throughout the evening, neighbors and those involved in the renovation project lamented the destruction of the mansion, which several called the architectural cornerstone of the area.

"It's a very tragic loss," said one of the architects for the renovation, who asked not to be identified. The building is owned by Scott Mcleod, who could not be reached for comment.

Fire officials said the last member of the renovating crew had left the building about 4:30 p.m., about two hours before the fire broke out.

Built for Capt. Frederick Augustus Miller, a Union Navy veteran of the Civil War, the house was designed for a single family and featured a magnificent stairway and numerous nautical motifs. The architect was Paul J. Pelz, who also designed the Library of Congress.

According to "The City of Washington, An Illustrated History," Miller and his wife had both inherited family fortunes and could have lived anywhere, but Miller was a staunch believer that Washington was becoming one of the world's loveliest cities and a center of culture.

Renovators said that most of the original features of the house were intact before the fire, despite the building's having been turned into a 35-unit rooming house called the Argyle Guest House during World War II.

"Stone shells adorn the exterior frieze" . . . and the "dining room mantlepiece is carved with dolphins, neptunes, tridents and more shells," according to the book.

Another outstanding feature of the house was a massive "stained glass painting" honoring Miller's naval hero, Adm. David G. Farragut. Located in the dining room and called the Farragut or Admiral's window, it included images of fish and deep sea flora, according to the book.

However, several neighbors noted that the fire spared a unique feature of the site -- the first "automobile house," or private garage, in the District. It was designed to house Miller's electric car.

Also contributing to this story were staff writers Robert Galano, Lyle V. Harris and Michel Marriott.