A police officer packs up the frames of the stolen paintings outside the Paris Museum of Modern Art on Thursday. (Jacques Brinon/Associated Press)

The thief or thieves behind the brazen theft of artworks by the likes of Pablo Picasso and Amedeo Modigliani in Paris profited from a broken alarm system, the mayor of Paris revealed on Thursday afternoon.

In a statement, Mayor Bertrand Delanoe confirmed that the alarm system at the city-owned Paris Museum of Modern Art had been broken in several rooms since March 30.

Though the security system operator had ordered spare parts for the repair, the equipment had not yet arrived from the supplier, the statement said.

The museum reopened in 2006 after a costly security system upgrade that took three years.

Overnight theft

Paris police revealed on Thursday morning that five valuable canvases were snatched from the museum.

Video surveillance cameras captured footage of a single masked intruder entering the museum, located in the city's tourist-friendly 16th arrondissement across the Seine River from the Eiffel Tower.

The five paintings taken were:

• Le pigeon aux petits-pois (The Pigeon with the Peas) by Pablo Picasso.
• La Pastorale (Pastoral) by Henri Matisse.
• L'olivier pres de l'Estaque (Olive Tree near Estaque) by Georges Braque.
• La femme a l'eventail (Woman with a Fan) by Amedeo Modigliani.
• Nature-mort aux chandeliers (Still Life with Chandeliers) by Fernand Léger.

The Paris prosecutor's office initially estimated the value of the five paintings as high as 500 million euros (about $620 million).

However, Christophe Girard, deputy culture secretary at Paris's city hall, later said the total value of the works is "just under 100 million euros [about $124 million]."

The museum was closed Thursday as police gathered evidence and continue their investigation.

Security failure

Though three guards were on duty at the museum overnight, they "saw nothing," Girard said.

Police check out the broken window at the museum, where they believe a lone masked thief broke in. (Bertrand Guay/AFP/Getty Images)

The museum's security system was disabled and it appears the intruder entered by shearing off a padlock on a gate and breaking a window.

Earlier on Thursday, Delanoe released a statement saying he is "saddened and shocked by this theft, which is an intolerable attack on Paris's universal cultural heritage."

Palais de Tokyo is another contemporary art museum located next door to the Paris Museum of Modern Art. Its director, Pierre Cornette de Saint-Cyr, condemned the thief or thieves.

"You cannot do anything with these paintings. All countries in the world are aware, and no collector is stupid enough to buy a painting that, one, he can't show to other collectors, and two, risks sending him to prison," Cornette de Saint-Cyr told a local television station.

"These five paintings are unsellable, so thieves, sirs, you are imbeciles. Now return them."

Ransom demand likely: Art Loss Register

Ideally, the Paris police will be able to apprehend the thief or thieves quickly and secure the return of the artwork.

However, the museum will likely be getting a ransom demand in the next few days, according to Chris Marinello, executive director and general counsel of Art Loss Register, an international organization that maintains a vast database detailing stolen art worldwide.

"A theft of this magnitude had to be done by a very sophisticated group," he told CBC News Thursday morning.

"They managed to disable a major French museum's security system and leave with five major works of art.… There are gangs out there that specialize in this sort of thing."

Once the thieves realize the open market is out of the question, "they're going to go down deep into the black market where art of this calibre is often traded for weapons [or] is used in international terrorism. It obtains a sort of 10 to 20 per cent value of its true value in the black market," Marinello said.

Some type of intermediary will likely demand money from the museum or from its insurance company, he said, adding that the Art Loss Registry discourages paying ransoms because it encourages more theft and shows "that crime does pay."

Time is also of the essence in the investigation and recovery effort, Marinello said.

"If we don't recover these in the next several months, you're going to find that it may take years before they resurface."