The Dutch art gallery at the centre of an audacious heist last week said that the electronic locks on its doors were disarmed before thieves made off with paintings by Picasso, Monet and others worth tens of millions of dollars.
The seven stolen works - which also include a Gauguin, a Lucian Freud and a Meyer de Haan - were on loan from a private collection for a special exhibition at Rotterdam's Kunsthal of impressionism, expressionism, and other modern art movements.
The theft was one of the art world's most dramatic in recent years, and one of the biggest in the Netherlands.
The apparent ease with which the thieves entered and escaped has raised questions about the Kunsthal's security system and whether an insider was involved.
The Kunsthal said in a statement on Monday that the electronic locks on its doors were in working order, but were designed to automatically unbolt shortly after the burglar alarm was set off. After that, only mechanical door locks stood between the intruders and the Kunsthal's treasures.
"The theft on Monday night suggests the intruders forced the lock after the unbolting, presumably quickly," the statement said.
The thieves forced the mechanical lock on an emergency exit at the rear of the ground floor gallery. Police arrived at the scene within five minutes, but the intruders had already gone.
Defending its security installation in the face of speculation, the gallery said an independent inspection of its security apparatus in August had found only that one motion detector was blocked off. It had been immediately repaired.
On Friday, the police released security camera footage which showed the thieves entering through the back door before disappearing from the camera's field of view. Seconds later, they come back into view carrying bulky objects, and leave the same way they entered.
The faces of the thieves cannot be seen clearly on the footage, but police are appealing to the public to come forward if they recognise their bags or clothing. Police say they have already received 60 tips relating to the robbery.
The Kunsthal, in Rotterdam's leafy museum quarter, does not have its own collection.
It had only just opened a new exhibition of works from the private Triton Foundation - collected by the Cordia family, which made its money in shipping and oil - to celebrate its 20th anniversary when the break-in took place during Monday night.
The gallery reopened on Thursday, and the empty spaces previously occupied by the stolen paintings were filled with new works from the Triton collection.
Police appeals for information are posted around the museum park, home to half a dozen museums besides the Kunsthal, which was designed by Rem Koolhaas.
The museum has not said which paintings now hang in place of the stolen works, which included Matisse's "La Liseuse en Blanc et Jaune", Monet's "Waterloo Bridge, London" and "Charing Cross Bridge, London", Gauguin's "Femme devant une fenêtre ouverte", De Haan's "Autoportrait" and Freud's "Woman with Eyes Closed".
But even the absence of Picasso's "Tête d'Arlequin", which was the exhibition's centrepiece, has not dampened interest in the exhibition, which was crowded on Sunday.
"Numbers are way up since the theft," said the cloakroom attendent with a smile. "Disaster tourists."
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