In $6 million art heist, the question is why. Only the unwary or unscrupulous will buy works like these

How this week's $6 million art theft - New York City's largest ever - was accomplished is clear, but precisely why it was carried out is another matter. The burglars - the police estimate there were at least two - entered Colnaghi Ltd., a prestigious Upper East Side art gallery specializing in Old Master painting and drawings, sometime during the evening Monday. They smashed through a glass skylight on the roof of the building at 26 East 80th Street, then dropped into the gallery itself.

When they left - hurriedly, one assumes, since they accidentally tripped an alarm - through a rooftop hatch, they took with them 19 Old Master paintings and 10 drawings worth at least $6 million.

What they took was small and choice: two paintings by Fra Angelico, one of the outstanding Italian artists of the 15th century, as well as works by Ludovico Carracci, Jean-Baptiste Chardin, Villem Kalf, Giovanni Castiglione, Gaspard Dughet, and various other, lesser-known Italian and Dutch masters dating back to the 14th century.

Why they took them, however, remains a mystery.

Thanks to such organizations as the International Foundation for Art Research and Interpol - which distribute information on stolen art to local police, the FBI, and various international law-enforcement agencies - dealers, curators, and collectors around the world will know in a matter of days precisely what was stolen and how to identify every one of the pieces.

The thieves, as a result, will be forced to dispose of their loot with unscrupulous collectors who are fully aware that they are dealing with stolen goods, or to attempt to sell the lesser-known items to unsuspecting buyers sometime in the future. Neither seems very likely.

Of course, art can also be stolen and held for ransom - either for money or for political advantage.

That also, under the circumstances, seems unlikely. If ransom was the motive, why take so many paintings and drawings, including several of relatively little interest and value? Why not steal one or two sensational, really newsworthy items, a Van Gogh perhaps, or a C'ezanne, and demand ransom? After all, by today's auction standards, $6 million for 29 Old Master paintings and drawings isn't really very much.

Naturally, there's the matter of availability, to say nothing of accessibilitiy. Important works of art simply don't lie around galleries waiting to be stolen. Security measures are generally too strict, for one thing. And for another, paintings of that caliber are rare enough in public museums, let alone in commercial galleries. On the other hand, the reputation of Colnaghi Ltd. is such that any knowledgeable thief could simply have assumed that something of considerable, if not truly great, value could always be found there.

No matter who the burglars were, however, and whatever their motive, it seems fairly clear that they knew what they were doing when they broke into the gallery, chose the works they took, and stripped several of the paintings of their frames to facilitate their escape.

To date, however, no one at the gallery or elsewhere is willing to discuss the burglars' possible level of sophistication, or whether the thieves have attempted to contact the owners. When asked about the latter, Elizabeth Hayt-Atkins, director of the Art Theft Archives of the International Foundation for Art Research, responded, ``Not as far as we know.''

Theodore F. Wolff is the Monitor's art critic.