NEW YORK -- A pair of agile amateurs probably was responsible for Monday night's $6 million art heist that stripped a prestigious Manhattan gallery of 27 works by old masters, police said yesterday.

The police department's top art crimes detective said at least two burglars carried out "a hairy operation" by smashing through a skylight on the roof of a prestigious gallery on Manhattan's East Side and lowering themselves to the floor of the gallery.

Detective Thomas Moscardini also speculated that the thieves were not professionals, although they made off with $6 million worth of paintings and drawings.

"They did leave numerous valuable pieces behind," he said. "They took what they could carry and they were valuable paintings. But there were still many valuable things left around."

The detective said police were "checking out some leads" and had alerted Interpol and the FBI to keep a close watch on international borders.

The paintings were stolen from the Colnaghi Gallery in Manhattan sometime Monday night in a heist considered to be one of the biggest ever in the city.

Among the 18 paintings and nine drawings stolen were two paintings by Italian Renaissance master Fra Angelico. The two works -- "Sts. Francis and Nicholas" and "Sts. Dominic and John the Baptist" -- are worth a total of $4 million.

Capt. William Greeley of the Safe and Lock Squad said the thieves stole works from the gallery storeroom and its display walls, trampling and damaging two pictures in the process.

"They went through the skylight, went in and did some ransacking and could have been looking for money," he said.

Greeley, asked if there was any suspicion the heist was an inside job, said "None whatsoever."

Moscardini, an expert in art theft, said there was little chance of recovering the works, since statistics show a 14 to 18 percent recovery rate.

Gallery managers refused to discuss the theft but said arrangements to offer a reward for the return of the paintings were under discussion with their insurance company. They refused to disclose potential terms or the insurer's name.

The London-based gallery, established in 1760, is distinguished as a dealer in works by the old masters, generally considered the great European painters up to the 18th century. Its New York gallery opened in 1982.

Major art heists of the century have included a January 1976 theft of 118 Picassos, many considered priceless, from the Palais des Papes in Avignon, France. On Oct. 27, 1985, nine paintings worth at least $10 million, and all by or related to French impressionist Claude Monet, were stolen by five armed thieves from the Marmottan museum of the Academy of Fine Arts in Paris.

On May 21, 1986, 17 old-master paintings -- including works by Vermeer, Goya, Rubens, Vela'zquez and Gainsborough valued at at least $40 million -- were taken from the mansion of Sir Alfred Beit near Dublin.