Key West, Florida (CNN) -- For more than 20 years, the bulletproof museum case housed a small piece of yesteryear: a gold bar recovered from a sunken Spanish galleon. Today, its case is broken, littered with black fingerprint dust. The treasure is gone. Stolen. Two thieves were caught in the act by the museum's security cameras.
"This is a special piece," said Melissa Kendrick, executive director of the Mel Fisher Maritime Museum in Key West, Florida.
"All the pieces have an incredible historic value, but this is the piece that was shared with the public in a whole totally different way."
It was different because visitors could touch it. By reaching into the specially designed display case, more than 6 million people have touched the 74.85-ounce bar, valued at more than $550,000.
"They're touching something that belonged to someone in 1622," said Carol Shaughnessy, author of "Diving Into Glory."
"Ordinarily people don't get to touch something like that. You can't touch an Egyptian mummy. This is a hands-on connection to history."
But now, what does a thief do with a priceless, high-profile artifact? Is there an underground market that will pay $550,000 for this almost 400-year-old piece of solid gold? One expert says no.
"That's why these crimes don't make a whole lot of money for the criminals," said Robert Wittman, a former FBI agent who once headed the FBI's Art Crime Team.
"It doesn't make sense to do it."
Wandering through the museum, the thieves can be seen in security video trying to open museum doors. The video is incredibly clear. First, they appeared to be targeting a display case of gold chains. Then, after a security guard left this part of the museum, a man can be seen reaching into the case housing the gold bar and placing the little piece of history into his pocket before exiting the museum.
"We're getting information and following leads," said Key West Police Chief Donie Lee.
"Unfortunately we haven't got the best lead, which is, I know that person and we go out, and it's a positive ID, and we're able to go out and pick those guys up."
What makes the crime so shocking, police said, is that the thieves were able to snap the glass at its edges. It's not just any glass, but three-eighths-inch thick bulletproof Lexan glass.
"By designating this as a handling object, it brought certain risks to the bar," Kendrick said.
"But after your first five, and your next 10, and when you get to 25 years, you start to get to the point when you think that it's never going to happen."
Treasure hunter and salvor Mel Fisher recovered the solid gold bar from the wreck of the Santa Margarita in 1980. Fisher and his team had been searching for the Nuestra Senora de Atocha and instead found the sister ship, the Santa Margarita. Both ships had gone down in a hurricane off Key West shortly after leaving Havana, Cuba, in 1622.
The ships were headed home to Spain with a cargo of gold, silver and coins from the new world.
The team found the Atocha in 1985. The stolen bar is one of dozens of gold and silver bars retrieved from the bottom of the sea.
Experts say that about 90 percent of stolen art and artifacts is eventually recovered that but it often takes years to find. The FBI has recovered more than 2,600 items of cultural property valued at more than $142 million. The items range from Colombian artifacts to Rembrandt paintings.
Wittman, the former FBI agent and author of "Priceless: How I Went Undercover to Rescue the World's Stolen Treasures," said the market is incredibly small for these high-profile objects. He said thieves often steal the items and then try to figure how to sell them.
"We recovered paintings and artifacts that were missing for many years. Ten, 15, sometimes 20 years, because the thieves couldn't get rid of them," he said.
"They kept them in their closets. They were white elephants. They made no money out of the deals. They were stuck."
In 1990, thieves entered the Isabella Stewart Gardner Museum in Boston, Massachusetts, and stole 13 works of art, including three Rembrandts from the 1600s. None has been recovered, and federal agents are using DNA to try to find the perpetrators.
Wittman said no legitimate collector would take the risks associated with buying stolen goods.
"They don't buy stolen property, because ... they can't show it, they can't enjoy it. ... It makes them into criminals, and the last thing they want to do is spend a lot of money for a painting or for an artifact, whether it's gold or whatever, and have it seized by the police and go to jail," he said.
Key West authorities said they believe the thieves were not locals and that they are probably long gone. The museum's insurance company is offering a $25,000 reward for information leading to the return of the bar.
Police said they remain hopeful they will solve the crime but just hope they can recover this golden piece of history.
"This is going to end up in somebody's house probably, used as a paperweight," said Lee, who is leading the investigation.
"Other than melting it down, which is the worst-case scenario for everyone, we're just hoping that they will come to their senses somehow and return this back to the museum."