Whoever broke into the Kunsthal Museum in Rotterdam in the wee hours of Tuesday morning didn’t have to get past the guards to steal famous masterpieces by Pablo Picasso, Claude Monet, and Henri Matisse—because there weren’t any on duty.

When the alarm bells rang as the thieves made off with seven paintings worth over €50 million (about $65 million), it took security officials just five minutes to respond to the call, but it was enough time for the robbers to make a safe getaway. The thieves obviously knew the security at the Kunsthal was an automated unmanned system, but they also knew enough about how to get around the high-tech features to lead inspectors to wonder if the heist was an inside job. Or if it could have been done by someone who was highly aware of how the specific security system worked—and could have failed, as it did during this robbery. “The alarm system in the Kunsthal was supposed to be state-of-the-art,” Rotterdam police inspector Roland Ekkers said at a news conference. “But somehow the people responsible for this found a way in and a way out and they found time to take seven paintings.”

Police are now hoping old-fashioned close-circuit cameras will provide vital clues to just who broke in and how they were able to take so many paintings in such a short time. The stolen paintings included Picasso’s 1971 Harlequin Head and Henri Mattise’s 1919 Reading Girl in White and Yellow. They were part of an exhibit of 150 pieces from a private collection at the museum.

On Tuesday, Kunsthal director Emily Ansenk said she could not understand how the museum’s state-of-the-art security system had failed. It should have detected any movement into the building, but it was only tripped when the thieves left. “It is a nightmare for any museum director,” she told reporters. Interpol has listed the paintings on its international register of stolen art.

The Interpol listing will render the works virtually impossible to sell on the open market, says Christopher Marinello of the Art Loss Register, which helps museums and collectors recover stolen treasures. “Paintings like this are less valuable once they are stolen—unless the thieves are planning to ask for ransom or if they were stolen ‘to order’ by an art collector,” Marinello said.

Art theft is the third-most-lucrative sinister crime—following only drugs and weapons dealing. But less than 15 percent of all stolen art is ever recovered, according to the Art Loss Register, and some of it takes decades to find. In a landmark case in Rome this spring, 37 rare paintings were recovered after a whopping 40 years. In a strange coincidence, they were found in a private home just a few blocks from where they were stolen. In July, a Florida couple tried to sell Matisse’s Odalisque in Red Pants, which had been swapped for a fake a decade earlier at the Contemporary Art Museum in Caracas, Venezuela.

The paintings lifted from the Kunsthal were from the Triton Foundation’s private collection, which had been on loan to the Kunsthal since Oct. 7. The Kunsthal has no permanent collection of its own, but is a showcase for borrowed works—and investigators are focusing on whether the Triton Foundation will be targeted for ransom. The Kunsthal exhibit is the only time the foundation has ever exhibited its entire collection at one museum. While only seven of 150 works were taken, they were among the most valuable, implying that the thieves knew as much about art as they did about security.