Paris ? In a stealthy overnight heist, burglars slipped into the Paris apartment of Picasso’s granddaughter and spirited away two portraits of women the artist loved, slicing one of the paintings out of its frame.

The thieves were so quiet that the two people in the apartment of Diana Widmaier-Picasso at the time didn’t hear them make off with the art treasures, police said. The burglars left few clues, and police said they were not sure how the intruders gained entry.

The two paintings – one of Pablo Picasso’s daughter Maya, the other of his second wife Jacqueline – together are worth an estimated $66 million.

The paintings join 549 other missing or stolen works by the prolific Spanish painter, sculptor, graphic artist and ceramist, considered by many the leading artist of the 20th century. According to the Web site of the Metropolitan Museum of Art in New York, Picasso produced more than 20,000 works of art during his long career.

Art experts say that if the burglars hope to sell the paintings, they are in for a surprise.

Any work by Picasso is “very hard to fence because it’s so well-known – stealing a Picasso is like stealing a sign that says, ‘I’m a thief,”‘ said Jonathan Sazonoff, who runs a leading Web site on stolen art.

Katie Dugdale of the Art Loss Register, which maintains the world’s largest database on stolen, missing and looted art, said that although it’s difficult, famous artworks can be sold on the black market.

“Even though they can’t get full value, there’s still some value unfortunately,” she said, particularly if the artworks are used to fund other illegal activities, like arms trading.

In high-profile cases like the theft of the Picassos in Paris, recovery is likely because of intense media attention and ramped-up police efforts.