BOSTON — The FBI searched around the world Monday for a dozen priceless artworks stolen from the Gardner Museum. Authorities said it was the biggest theft of modern times — a $100 million-plus caper exceeded only by the looting of Germany’s national bank at the end of World War II.

The museum, meanwhile, disclosed that the missing works were insured for damage as well as restoration and conservation, but that it had no theft insurance.

“Insurance coverage for a collection as valuable as the Gardner’s would be prohibitive,” museum spokesman Corey Cronin said. “The (museum) board decided a number of years ago not to insure the collection since, even if an insurance policy paid off, the museum could not use the money to replace the items lost.”

FBI Agent Dennis O’Callaghan said he could not discuss any leads the FBI might have uncovered. He added, “There are sources around the world we would reach out for” in the effort to recover the works, which included well-known paintings by Rembrandt, Degas, Vermeer and Manet.

He said no ransom demands had been received, but if any comes, it would be treated like a kidnapping. He would not elaborate.

But O’Callaghan, of the bureau’s Boston office, was not optimistic that the art would be quickly recovered.

“Despite speculation,” he said, “the sad part is this may wind up in a vault somewhere. Unfortunately, on valuable art objects, the track record (on recovery) isn’t really good.”

In another wrinkle, the museum revealed that the will of its founder, Isabella Stewart Gardner, prohibits the museum from buying back the art for ransom, or from purchasing substitutes to install in their place.

The objects of the search are paintings, drawings and an ancient Chinese vase stolen around 1 a.m. Sunday from the museum established, designed and filled by Mrs. Gardner on Boston’s tree-lined Fenway.

Two men dressed as police officers persuaded two museum guards to let them inside, reportedly by telling the guards there was a disturbance in the area. They then overcame the guards, bound them with tape, disabled the security system and — after spending about two hours in the museum — made off with the loot.

A cleaning crew found the guards about six hours later.

Police put the value of the missing paintings at a minimum of $100 million. The museum said they were worth “hundreds of millions,” and most art experts agreed. No exact value can be put on the paintings because they have been not been on the market for nearly 100 years.

The Guinness Book of World Records lists the ransacking of the Reichsbank in April-May of 1945 as the most lucrative robbery in modern history, worth $3 billion in contemporary dollars. Guinness also notes that the Philippines government has accused former President Ferdinand Marcos and his wife, Imelda, with stealing between $5 billion and $10 billion during their tenure in power.

Probably the most celebrated art heist of all time was the 1911 theft of the Mona Lisa from the Louvre in Paris. It was recovered in Italy in 1913. Because no price has ever been established for the painting, the value of the theft has never been determined.

The thieves in Boston apparently entered a side entrance for service deliveries, where an archaic wooden door with a caged peephole leads into a security area. A camera suspended from an outside museum wall was trained on the entrance Monday. Museum officials did not say if the camera took the thieves’ picture.

Museum director Anne Hawley told reporters the guards were only doing their job and would not be disciplined.