BOSTON -- Inside the Isabella Stewart Gardener Museum, patterned yellow wallpaper still shows through the empty frames of irreplaceable masterpieces stolen 19 years ago from the museum.

NewsCenter 5’s Pam Cross reported that the heist, which was the biggest art theft in history, remains perhaps the most intriguing mystery from the city’s past. The museum has increased the reward for the safe return of the stolen paintings, which include works by Rembrandt and Vermeer, to $5 million.

“Someone, somewhere saw these paintings. Someone, somewhere knows who these thieves are,” said author Ulrich Boser at the museum Thursday. Boser’s new book fingers a Boston mobster as one of the culprits behind the notorious theft.

Boser’s investigation of FBI files and interview points to David Turner, a Boston mobster in jail for another crime, as a mastermind in the theft. Works stolen during the robbery include a painting by Rembrandt believed to be his only seascape, as well as a work by Vermeer.

“David Turner has never been charged with the crime and the statue of limitations for actually breaking into the museum have expired,” said Boser. “I want to make it clear that I have not cracked this case, I have not solved this case putting David Turner’s name out there.”

The hidden art may have been lost as the circle of thieves vanished following the March 1990 robbery. In total, 13 works were stolen from the museum, after two men dressed as Boston police officers gained entry to the building and subdued guards.

“One of the reasons this has become such a confusing and such a mystery of a case is that a number of the people who’ve been linked to the case over the years have passed away,” said Boser.

Boser created a Web site called Open Case dedicated to the Gardner Heist and other crimes, in the hopes that members of the public may be able to provide new information about unsolved cases.

The journalist and author said that art theft has become a major black market enterprise in the years following the crime.

“Experts believe it’s between a $4 billion to $6 billion business,” said Boser. “That makes it one of the largest criminal activities in the world,” he said.