Two men in black ski masks entered a museum in Oslo yesterday and, as terrified tourists watched, brazenly pulled from the wall two masterpieces by the Norwegian painter Edvard Munch, including "The Scream," one of the most familiar images in Western art.

After threatening a guard at the Munch Museum with a handgun and shouting in Norwegian, the thieves exited with the paintings still in their wooden frames. They escaped in a black Audi A6 driven by a third man. Police found the frames discarded less than half a mile away, and the Audi abandoned at a nearby tennis court.

"It was a very rude crime," said Jorn Jurgesen, a spokesman for the Oslo police department. "These two paintings were two things of our heritage treasure, so now they are taken and we will try to do the best we can to find them."

It was the second time in a decade that a version of Munch's iconic, head-clutching figure of howling alienation -- he painted the nightmare vision four times -- has been stolen. The best-known rendering of "The Scream" was grabbed in 1994 from the National Gallery in Oslo. It was recovered three months later, after the thieves sought to collect a ransom. Two men were arrested.

"The Scream" stolen yesterday from the Munch Museum was made with tempera on board. It has the familiar haunted -- and haunting -- wide-eyed, open-mouthed figure transfixed beneath a bloody sky. The other painting taken yesterday, "Madonna," is part of a series Munch painted with "The Scream" in 1893-94. It shows a nude woman swooning under a red halo.

Munch, who lived from 1863 to 1944, said his inspiration for "The Scream" came one night while walking on a bridge above the Oslo fjord. The water was dark, the clouds red as blood and fire. "Alone and trembling with fear," he recalled, "I experienced nature's great scream."

Almost as soon as the artist completed the work, it became a much-reproduced and widely selling image -- first as a banner of existential dread, later as a campy, darkly funny poster and inflatable pop-up toy in dorm rooms and offices. Norway is immensely proud of it. Yesterday the nation was groping to make sense of how it could be stolen -- twice.

"People are frightened and scared and they are wondering what this is and how this can go on," Jurgesen said. "The whole police [force] in Oslo are now working with this case."

The bandits -- wearing black masks that Norwegians call "Finland caps" -- entered the museum at 11:10 a.m. Oslo time, when some 50 to 70 people were visiting. One of the men shouted and seemed angry, threatening to damage the paintings, according to police. He pointed a handgun at a female security guard, but she was not hurt. Another guard was also present.

"I saw one of the men put a gun right behind a guard's head," Richard Marcus, a 63-year-old Texas businessman, told Reuters. "It took a long time for the police to come."

Jurgesen said officers arrived within 15 minutes.

"The paintings were simply attached by wire to the walls," Francois Castang, a radio producer, told France Inter radio, according to the Associated Press. "All you had to do is pull on the painting hard for the cord to break loose -- which is what I saw one of the thieves doing."

Jurgesen confirmed that only wires secured the paintings, but said there also was a silent alarm.

Gunnar Soerensen, head of the Munch Museum, denied that security was too lax, according to Reuters. He said measures such as automatically locking doors when paintings are pulled off walls could endanger people.

Police are analyzing evidence gathered at the crime scene, but would not characterize that evidence.

Art experts estimate "The Scream" could be worth more than $70 million. But it would be impossible to sell on the open market.

"Maybe there are some sick people who would have joy hanging one or two of these paintings on the wall," Jurgesen said.

The other possibility, he said, is that police will shortly receive a ransom demand. That's what happened after "The Scream" was stolen on the opening day of the 1994 Winter Olympics in Lillehammer. Police say they do not think that crime and yesterday's theft are connected.

In the earlier case, the thieves climbed a ladder and broke through a second-story window, then used wire cutters to get the painting down. They reportedly left a note: "Thanks for the poor security." About three weeks later, police received a demand for ransom of $1 million. The robbers were caught after a sting operation in which an intermediary pretended to offer money for the painting's safety.

Jurgesen predicted the latest case of the stolen "Scream" will prompt some national soul-searching: "There must be some discussion after this in the municipality, or the police, or the museum or the art society of Norway: How can this happen?"

Two men enter a getaway car in Oslo, Norway, after loading the stolen paintings, including "The Scream," left, into the back.Police discovered these frames in the street less than a mile from the museum, and the getaway car nearby.Employees carry Edward Munch's "Madonna" into a Christie's showroom in Munich in 1999. Yesterday museum visitors looked on as armed thieves ripped it from the wall in an Oslo, Norway, museum.Police technicians secure an area of the Munch Museum after the heist. Whoever stole "The Scream" would find it impossible to sell, art experts said.