BOSTON, MARCH 19 -- A stunning theft of Dutch masters and modern French paintings from the Isabella Stewart Gardner Museum touched off a worldwide investigation and left art authorities puzzled about a motive.

The well-executed theft, discovered early Sunday morning, involved a dozen works of art, including an exceedingly rare Vermeer painting, three Rembrandts and works by Manet and Degas as well as a Chinese bronze beaker.

Several experts described the heist as a thoroughly professional operation that would rank as the most lucrative art theft in world history, worth at least $200 million, if there were a ready market for the stolen objects. Most of the pictures, however, are so well known that they are virtually impossible to sell.

"A part of our heritage has been stolen from us. It is a barbaric act," Anne Hawley, the Gardner's director, told reporters at a press conference in the museum's courtyard. "We are confident that these works will be recovered."

The bold theft sent shudders through the staff at the Boston Museum of Fine Arts, just two blocks from the Gardner, which is showing a blockbuster exhibit of Monet's "series paintings" from the 1890s. The MFA's security chief promised to tighten procedures but said there is an inevitable tension between the desire to exhibit and the need to protect.

Officials at the FBI, who are coordinating the international investigation, refused to divulge details of the museum's security system or the hunt for the thieves. Museum officials said they had received no ransom messages as of late today.

The museum, which is closed on Mondays and plans to reopen to the public at noon Wednesday, was hit by two apparently knowledgeable thieves who entered by a side door during the night watch, bound the two guards on duty and removed the artworks from three exhibit areas on the first and second floors.

"It's devastating to me," said Hawley, who has headed the museum for the past six months, describing her feelings on seeing the Dutch Room stripped of its Vermeer and Rembrandts. "It's as if I experienced the death of a dearest friend."

Karen Haas, the museum's acting curator, said it seemed "very clear" that the thieves knew exactly which artworks they wanted, since they left behind many stunning paintings and objects. She said the walls would be left blank where the missing pieces belong.

Director Hawley confirmed police reports that the two guards opened the side door because the thieves were posing as Boston police officers. She said the guards were not seriously hurt and that they would remain on the museum staff.

The motive for the theft remained a mystery. Experts considered ransom the most likely possibility, saying that museums and insurance companies are known to quietly deal with thieves.

Opinion was divided, however, on whether the art objects were stolen to order. Some authorities said the choice of certain objects over others suggested that there was a "hit list" of works stolen on commission from a wealthy individual for a private collection, but others discount that theory.

"Every time there's a thief, there's a James Bond theory," said John Walsh, director of the J. Paul Getty Museum in Los Angeles. "So far, we've never found the demented billionaire on a submarine off Uruguay, or wherever he is supposed to be. I just wish I could explain the things that didn't reappear."

The Gardner Museum carried some insurance on its collection, but only against damage. There was no theft insurance, according to museum spokesman Barry Wanger, because Isabella Stewart Gardner, the wealthy widow who created the museum, specified in her will that the facility could never buy or substitute works of art.

It is a common practice for museums to carry no insurance on their collections, according to James Mooney, president of the Inland Marine Underwriters Association, a New York-based trade group representing 365 insurance companies that write most of the "special risk" policies in the country.

Some museum directors purchase theft insurance, but not for the entire value of their holdings. Instead, Mooney said, they "pick a figure out of the air" and buy enough insurance to keep the institution operating in case of a major theft.

Many museums carry no insurance on their collections, according to experts, because the objects are irreplaceable and many museum directors prefer to spend the tens of thousands they would need for premiums on salaries for extra guards.

Mooney said the Gardner theft was a spectacular peak in an upward trend in art theft in recent decades. He said it would probably make museum insurance more expensive and more difficult to obtain but would not eliminate the practice.

The theft came as a shock to museum directors worldwide, many of whom called the Gardner today to offer support to the director. One of the most grieved was Walsh at the Getty Museum, an authority on Rembrandt and a former curator of paintings at Boston's Museum of Fine Arts.

"It is the biggest art theft I have ever come across," Walsh said in an interview, adding that when he read of the theft it felt "like being kidnapped."

He said the thieves were either "crazy and ignorant and wanted to sell them" or else were planning a ransom scheme.

"What's sobering here is that the Gardner Museum is an extremely well-run place," Walsh said. "They did get great masterpieces, but they got rather large and unwieldy objects. It doesn't make any sense."