Authorities were investigating the cause of a suspicious fire that swept through a historic, century-old loft building, destroying millions of dollars worth of art and at least eight galleries.

All that remained Monday of the six-story, block-square warehouse designed by the noted Chicago architect Louis Sullivan was a brick shell.Witnesses said Saturday's fire began in at least two spots, "and very seldom do two fires break out simultaneously," said Fire Department spokesman Tom Sheppard.

Gallery owners said the River North art district should recover quickly from the fire.

"The major galleries will be back in business," said dealer William van Straaten, who lost $3 million in prints in the blaze. "We'll be open Monday morning, at least on the phone."

The fire struck as dealers were preparing for the annual Art Expo spring exposition next month at Chicago's Navy Pier, which attracts art dealers from around the world and is considered the most important annual show for Chicago's art community.

Van Straaten estimated that the combined loss to all the galleries in the building could total as much as $50 million.

"I am totally wiped out," said Jonathan Santlofer, a New York artist who spent two years on the 13 paintings and drawings he had hanging in one of the galleries when the fire struck.

"I may try to do something like it again, but I won't be able to do it exactly the same," said Santlofer, whose abstract paintings used religious motifs.

The building was constructed in 1885 and had been recommended for landmark status by the Commission on Chicago Landmarks. In recent years, it was the linchpin of the burgeoning River North area, which is filled with restaurants, nightclubs and dozens of galleries.