From stolen Rembrandt paintings to drawings by Degas and Picasso, the world's most famous artwork has been victim to theft throughout history. While the vast majority of works have been recovered and returned to their proper owners, thousands of valuable items remain missing in action. Here are some of the most notorious examples.
It was early morning in March 1990 when two men carried out the largest art heist in history -- a brazen theft of masterpieces by such painters as Rembrandt, Vermeer and Manet valued at $580 million from a Boston museum.
Nearly three decades later, an investigator independently working the case said he's "100 percent convinced" the most valuable collection of stolen artwork is somewhere in Ireland, specifically in the possession of associates with the Irish Republican Army, or the IRA.
"I have been talking with several former IRA members -- individuals I've built a trust with over the years," art investigator Arthur Brand told Fox News on Wednesday.
"I'm convinced they are there," Brand said of the artworks. "The Ireland angle has been one of the most promising leads from the beginning."
But Brand's theory is one the museum said it's already probed extensively, concluding long ago the stolen artwork is not in Ireland.
"We have investigated leads related to the IRA thoroughly," Anthony Amore, the museum's chief investigator, told Fox News.
"There's not any current evidence that indicates our paintings are being held by the IRA, or in Ireland," Amore said. But, he added, "We are always happy to hear new information."
Just after midnight on March 18, 1990, two men dressed as police officers buzzed the side door at the Isabella Stewart Gardner Museum in Boston and claimed they were there to investigate a disturbance.
A little more than an hour later, the men left with a collection of artwork valued at around half-a-billion dollars -- 13 masterpieces including Rembrandt's only seascape, "The Storm on the Sea of Galilee," and Vermeer's "The Concert," a masterpiece said to be worth more than $200 million.
The thieves also snatched an ancient Chinese bronze beaker or "Ku" from the Shang Dynasty and a finial that once stood atop a flag from Napoleon's Army.
The suspects, described as white men in their 30s, convinced two inexperienced security guards that they were police officers responding to a call, before overtaking the guards and tying them up. They spent 81 minutes inside the museum, walking the dark hallways before making their way to the Dutch Room, where the most valuable works were found.
Brand, who is based in Amsterdam, is credited with helping German police track down bronze horse statues that stood in front of Adolf Hitler's Grand Chancellery building, as well as recovering Salvador Dali's "Adolescence."
The FBI declined to comment Wednesday on Brand's assertions, citing an ongoing investigation.
FBI Special Agent Geoff Kelly previously told Fox News the suspects had "inside knowledge" of the museum's surveillance system. In 2013, the FBI announced that it knows the identities of the thieves but claimed it would be “imprudent” to disclose their names.
Investigators said the individuals belong to a criminal organization, and they said they believe the art work has changed hands several times over the years. The agency said it believes the works were transported from Connecticut to Philadelphia where they were sold about a decade ago. From there, knowledge of the art works’ whereabouts is limited, the FBI has said.
The bureau’s investigation has focused largely on Robert "The Cook" Gentile, an 80-year-old Connecticut mobster who in May agreed to plead guilty to federal weapons charges and is currently imprisoned. Gentile has denied any involvement in the theft -- despite information collected by the FBI implicating him in the crime.
Gentile became a focus of the investigation in 2009 when the widow of Robert Guarente, another person of interest in the theft, told the FBI that Guarente and Gentile had at least two of the stolen works before Guarente's death from cancer in 2004.
Brand said Wednesday he believes it's unlikely Gentile knows where the Boston artwork is hidden.
"If he could have made a deal with authorities, he would have done it by now," said Brand. "He’s an old man now in jail for selling weapons."
Brand claims the IRA provided a "very, very strong lead" from the beginning of the investigation, noting the group's strong ties to the Irish community of Boston.
"The IRA is known to do such things," he said. "In 1974, in Ireland, they stole a few paintings and they tried to use it as exchange for getting imprisoned IRA members to be moved from England to Belfast."
Brand described the collection of art stolen from the Isabella Stewart Gardner Museum as "priceless" but said such works are often used as payment for drug deals or arms deals on the black market.
"You cannot put a price on this art when it comes to its historical value," he said. "Vermeer is considered one of the best artists of all time. He only made 34 paintings in his life so you can see if they steal one, it’s quite a loss. And Rembrandt's "The Storm on the Sea of Galilee" is the only seaside he ever made."
"These are works that are not only important for art lovers but for history," Brand said.
In May, the museum and its board of trustees doubled the reward for the 13 pieces of stolen artwork to $10 million. The increased reward will expire at the end of 2017.
Brand said that while the hefty price might crack the case, he said officials must be clear on the exact terms of the reward.
"These criminals don't trust their own mothers," he said. "You have to be open and clear. You have to tell them exactly what to expect."
"Maybe they are not all together anymore," Brand said. "What if someone is in possession of three of the pieces and they're in poor condition?"
"I have no idea if immunity is granted and by whom and under what conditions. Authorities must be clear on this if they're hoping for a break," Brand said.
Amore, meanwhile, said the museum has been very clear about the terms of its reward. Any criminal prosecution is determined by federal authorities.
"The museum for more than a quarter of a century has made it extremely clear about the conditions under which the reward will be paid," Amore said. "The museum is prepared to pay a fair share of the reward for less than the 13 paintings, if necessary."
Anyone with information on the whereabouts of the stolen artwork is urged to call 617-278-5114 or email tips to email@example.com.
Cristina Corbin is a Fox News reporter based in New York. Follow her on Twitter @CristinaCorbin.