Who solved these major art whodunits?

* The stolen $36 million Rembrandt postcard-size self-portrait on copper. Thieves escaped in a speedboat, blocking police by setting fire to cars on either side of the Swedish National Museum in Stockholm.

* The missing jewel-encrusted sword given to Civil War Gen. George G. Meade on behalf of Philadelphia citizens grateful for his Gettysburg victory on July 3, 1863.

* The world's second-largest crystal globe, once owned by the Empress Dowager of China in the 1850s, and stolen from a University of Pennsylvania Museum of Archaeology and Anthropology.

Answer: FBI Agent Robert K. Wittman, 52, a pre-eminent expert on art theft who retired yesterday. As the bureau's only undercover art-theft agent, he investigated cases here and in 18 countries.

Over 20 years, the son of antique dealers carved a new niche in the FBI's portfolio: art thefts, the world's fourth-largest crime after arms dealing, drug dealing and money laundering.

"It's one of the most unregulated businesses in the world, with no oversight," Wittman said. Deals are often consummated with "just a handshake."

Interpol estimates that $3 billion to $5 billion in art, artifacts and gems are stolen each year in the $200 billion business, $80 billion of which is in the U.S.

"The easy part is stealing it; the hard part is selling it," he said. But he said the thief usually can get only 10 percent of its worth.

Wittman's first case, with now-retired FBI agent Robert Bazin, involved the theft of Rodin's first major sculpture, "The Man with the Broken Nose," stolen from the Rodin Museum in Philadelphia by a gunman who fired a round in the wall and forced museum employees to lie down.

The FBI then sent him to the Barnes Foundation to study art, to the Gemological Institute of America to learn to rate diamonds like a jeweler and to the Zales Corp., in Dallas, to learn to identify rubies, sapphires, other gems and high-end watches.

Asked for his favorite case, Wittman quipped, "The next one is what gets you excited."

During an 11-year period, he recovered $245 million in art, artifacts and gems, and was the driving force behind the creation of the FBI's national art-crime squad, staffed with eight agents in 2005 and increased to eight this year.

In one three-year period, the squad recovered $130 million worth of art treasures.

Recoveries can have a big impact; 200 ceremonial artifacts stolen from Ecuador will be repatriated next Tuesday. Wittman also found another 500 stolen artifacts inside Ecuador.

But now, he's going to work at the Fox Rothschild law firm with former federal attorney Robert Goldman, who prosecuted art-related cases for 18 years with Wittman.

In a reprise of "Bob & Bob," as they call themselves, they plan to assist security directors in museums, galleries and auction houses to protect their collections and represent collectors.

"There's a huge exploding market ripe with fraud and forgery out there," said Wittman. *