A specialized FBI unit has recovered more than 20,000 stolen or fake art and memorabilia valued at over $900 million since 2004.

The bureau's Art Crime Program works behind the scenes to recover some of the world's most prized pieces that pit agents against dangerous, transnational organized crime groups, such as the Mafia and Russian organized crime, with tens of billions of dollars at stake.

Recoveries include Monet, Rembrandt and Dali paintings, an original Charles Dickens novel, Judy Garland's ruby slippers in "The Wizard of Oz," a 1952 World Series ring and 13th century Native American artifacts, among thousands of other items.

"Art and cultural property crime leads to billions of dollars in losses every year," Ellen Ferrante from the FBI’s Office of Public Affairs said during an FBI podcast released in November. "And art crime can involve a number of illegal activities, like theft, fraud, looting and trafficking of art across state or international lines."

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Kristin Koch, the supervisory special agent and program manager of the FBI’s Art Crime Program, said art crime involves all types of investigations, including stolen art from a museum, frauds and forgeries, antiquities trafficking and violations of Native American protection laws.

"Many art crime matters involve violations of federal law," Koch said. "The art crime market worldwide is a tens of billions of dollars industry every year in the illicit market. And the illicit market is also very large. So, whenever there's money involved, there's usually crime involved."

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Ferrante said FBI agents run up against organized crime groups from around the world during the investigations and recovery of these items. 

"Although tremendous strides have been made to combat cultural property crime, intelligence reveals this is a growing global threat, demanding proactive FBI measures and resources," Ferrante said. 

Koch added that organized criminal groups are largely responsible for trafficking stolen items from overseas to the U.S. and vice versa.

"There are a number of art crime investigations that are being conducted or have been conducted by the Art Crime Team that involve [transitional organized crime] TOC actors, such as the Mafia, Russian organized crime or other organized criminal groups throughout the United States," Koch said.

The "Wizard of Oz" slippers were stolen in 2005 and not recovered until more than a decade later in an FBI sting operation.

The slippers – of which only four authentic pairs remain – were on loan to the Judy Garland Museum in the late actress' hometown when someone climbed through a window in 2005 and broke the display case, prosecutors said. 

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At the time, the slippers were insured for $1 million, but the current market value is about $3.5 million.

Terry Jon Martin, 76, was indicted in May on one count of theft of a major artwork. He pleaded guilty in October and is awaiting sentencing.

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Another criminal who was brought down and ultimately convicted of selling fake Andy Warhol paintings is Massachusetts man Brian Walshe.

He was on house arrest for that case when he allegedly killed his wife, Ana Walshe, on New Year's Day. He pleaded not guilty to all charges.

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Koch referred to this case during the podcast, without naming Walshe, but explained the complexities of the case and how many figures were involved. 

"Through investigating that case, not only did we find that he had sold some fake paintings to the dealer in Los Angeles, but he had sold some fake paintings to a number of other individuals," Koch said. 

That included "an individual in France, another individual in Massachusetts and the paintings that he was using to make the fakes that he had basically stolen from a friend of his that was living in Korea."

The FBI did not respond to Fox News Digital's request for more information.

Fox News Digital's Emmett Jones contributed to this report.