"Olive tree near l'Estaque" painted in 1906 by Georges Braque.

Related: With Disabled Security System Thief Steals Five Paintings from Paris Museum of Modern Art

A broken alarm system made it as easy as 1-2-3: A masked intruder clipped a padlock, smashed a window and stole a Picasso, a Matisse and three other masterpieces from a Paris museum Thursday — a $123 million haul that is one of the world's biggest art heists. 

Offloading the artwork may prove a tougher task, however, with Interpol and collectors worldwide now on high alert.

In what seemed like an art thief's fantasy, the alarm system had been broken since March in parts of the Paris Museum of Modern Art, according to the city's mayor, Bertrand Delanoe.

The museum, in a tony neighborhood across the Seine River from the Eiffel Tower, reopened in 2006 after spending $18 million (euro15 million) and two years upgrading its security system. Spare parts had been ordered to fix the alarm but had not yet arrived, the mayor said in a statement.

So with no alarm to worry about, a lone masked intruder entered the museum about 3:50 a.m., said Christophe Girard, deputy culture secretary at Paris City Hall. The thief cut a padlock on a gate, then broke a side window and climbed inside — his movements caught on one of the museum's functioning cameras, according to the Paris prosecutor's office.

"Pastorale, Nympe et Faune" painted in 1906 by Henri Matisse.

The intruder later slipped back out, carrying the canvases and leaving behind empty frames. The whole thing took 15 minutes, a police official said.

Three security guards were on duty overnight, but "they saw nothing," Girard said. A night watchman discovered the theft around 7 a.m.

The stolen works included Pablo Picasso's "Le pigeon aux petits-pois" (The Pigeon with the Peas), an ochre-toned Cubist oil painting worth an estimated $28 million (euro23 million), and "La Pastorale" (Pastoral), a pastel-hued oil painting of nudes on a hillside by Henri Matisse worth about $17.5 million (euro15 million), Girard said.

Also seized were "La femme a l'eventail" (Woman with a Fan) by Amedeo Modigliani, "L'olivier pres de l'Estaque" (Olive Tree near Estaque) by Georges Braque and "Nature morte aux chandeliers" (Still Life with Chandeliers) by Fernand Leger.

Estimates of the total value of the paintings varied: The prosecutor's office initially put their worth as high as $613 million (euro500 million) but later downgraded the figure to about $111 million (euro90 million). Girard said the total value was about $123 million (euro100 million).

The broken alarm system also renewed concerns about museum security in the French capital. There was no operating surveillance system when a thief made off with a red sketchbook of 33 Picasso drawings from the nearby Picasso Museum while it was undergoing renovations last summer.

Within hours of Thursday's heist, red-and-white tape surrounded the Museum of Modern Art and signs on the Art Deco building's ornate bronze doors said it was closed for "technical reasons."

On a cordoned-off balcony, police wearing blue gloves and face masks examined the museum's broken window and the discarded frames. The paintings appeared to have been carefully removed from the disassembled frames, not sliced out.

Investigators were trying to determine whether the intruder was operating alone, Girard told reporters. Stephane Thefo, a specialist at Interpol who handles international art theft investigations, expressed doubt that one person could have pulled it off the heist, even if only one person was caught on camera.

Many high-profile art thefts have ended in failure, with the artworks recovered as thieves struggle to trade their illegal bounty for cash. But some famous stolen works remain at large — such as those seized more than two decades ago from Boston's Isabella Stewart Gardner Museum.

Alice Farren-Bradley of the Art Loss Registry in London said the Paris theft appeared to be one of the biggest art heists ever, considering the prominence of the artists, the value of the paintings and the high profile of the museum.

However, she said it will be "virtually impossible" to sell such prominent paintings on the open market and typically stolen art fetches lower prices on the black market.

"Very often they can be used as collateral to broker other deals" involving drugs or weapons, she said. "They are not necessarily going to be bought by some great lover of the arts."

Art expert Jean-Marie Baron said the heist could have been organized by thieves who plan to sell the paintings to wealthy individuals in Russia, China or elsewhere, and "who won't verify the origins of the paintings." Another possibility was that the thieves planned to "ransom" the paintings in exchange for a big insurance payout, he said.

The FBI estimates the market for stolen art at $6 billion. The Art Loss Register has tallied up to 170,000 pieces of stolen, missing and looted art and valuables.

Picasso is the world's most stolen artist due to his prolific output and the value of his works. The Art Loss Register lists some 550 missing Picasso pieces, including paintings, lithographs, drawings and ceramics, as of 2007.

Hours after Thursday's heist, the director of the neighboring Palais de Tokyo modern art museum called the thieves "imbeciles."

"Those paintings are absolutely unsellable. First off because these are very well known paintings. And also because we are in a new civilization ... of instant global communication," Pierre Cornette de Saint-Cyr told AP Television News. "The entire planet has pictures of these paintings."

The loss is "like the death of a family member," lamented Flemming Friborg, manager of Copenhagen's Glyptotek museum, known for its Impressionist collection.