The head of the Oslo gallery where armed robbers snatched Edvard Munch's The Scream has defended its security.
Few believe the thieves would try to sell the paintings on
Two masked men threatened a guard with a gun and tore the picture as well as Munch's Madonna from the walls of the Munch Museum in Norway on Sunday.
Gunnar Sorensen said a silent alarm went off and police arrived in minutes. "We think of security problems all the time and work with security," he said.
Insurers said The Scream was uninsured for theft because it was irreplaceable.
The paintings are thought to be too famous to be sold on the black market.
HOW THE SCREAM WAS STOLEN
1. Two masked men enter through the museum cafe
2. One man holds staff and visitors at gunpoint
3. The other man goes to the gallery and tears the Scream and Madonna from the walls
4. The two men make their escape, fleeing in a black Audi
Oslo police inspector Iver Stensrud said all resources were being used to search for the paintings.
"We are still working on new tips and are hoping for more," he told the state radio network NRK.
Norway's deputy culture minister, Yngve Slettholm, said he was horrified by the country's first armed art theft.
Major Norwegian newspaper Aftenposten carried the headline: "The world screams".
Mr Sorensen, head of the Munch Museum, told BBC Radio 4's Today programme a silent alarm went off the minute the raid started.
"To have bells ringing and things like that is not a very good device because of the people in the museum, and the robbers could have been stressed and that also could cause damage to the painting."
He said gallery staff had practised drills in case of a robbery.
He told Radio 4 it was a "serious loss" and they may never see this version of The Scream again.
The masked raiders were pictured on closed-circuit television
Three other versions of Munch's iconic image of a despairing figure exist - including one stolen in Oslo in 1994 but recovered several months later.
On that occasion, thieves set off an alarm - which was ignored by the guard - and left a note reading: "Thanks for the poor security."
Art recovery expert Tony Russell, who helped recover The Scream 10 years ago, told Radio 4 there was little galleries could do when faced with armed raiders.
"It's very difficult to have complete security when people are prepared to use violence of that nature," he said.
"I suspect there were other security measures taken [Mr Sorensen] won't discuss. I would be very surprised if there wasn't a sophisticated method of security."
The robbers were seen carrying the pictures to a getaway car
Thieves were likely to try to ransom it back to the gallery or government, or trade it in deals with other criminals, he said - but stealing to order does not happen.
Gallery bosses said they had reached a decision on how to deal with a ransom demand.
The Scream was insured for fire or water damage - but not for theft, said John Oyaas, managing director of Oslo Forsikring, the company that insures the city of Oslo's assets.
It did not have theft cover because even if the gallery got compensation, they could still not buy another one, he told BBC News Online.
The Scream - one of four versions completed by Munch in 1893 - has been valued at between $60m and $75m on the open market.
The robbers snatched two expressionist masterpieces
The BBC's David Sillito says the ghostly, screaming white figure in front of a red sunset is one of art's most familiar images - a symbol of despair and alienation.
In the aftermath of the raid, police cordoned off the museum, informed Interpol and alerted airports and border crossings.
The frames which had contained the two paintings were found smashed and discarded outside the museum while the gang's stolen black Audi getaway car was recovered a few kilometres away.
The Scream - painted not on canvas, but cardboard - may have been damaged by its removal from its frame.
The masked raiders were pictured on closed-circuit television.
They appeared to not know exactly where the paintings hung, crashed into a glass door on their way into the museum, and dropped the paintings twice on the ground before escaping.