Night watchman Richard Abath may have made the most costly mistake in art history on March 18, 1990, when he unlocked the doors of the Isabella Stewart Gardner Museum for two robbers who stole 13 works of art valued at more than $500 million. For years, investigators discounted the hapless Abath’s role in the unsolved crime. But, after 23 years of pursuing dead ends, investigators are focusing on intriguing evidence that suggests Abath might have been in on the crime all along.

But, after 23 years of pursuing dead ends, including a disappointing search of an alleged mobster's home last year, investigators are focusing on intriguing evidence that suggests the former night watchman might have been in on the crime all along -- or at least knows more about it than he has admitted.

Why, they ask, were Abath's footsteps the only ones picked up on motion detectors in a first floor gallery where one of the stolen paintings, by French impressionist Edouard Manet, was taken? And why did he open the side entrance to the museum minutes before the robbers rang the buzzer to get in? Was he signaling to them that he was prepared for the robbery to begin? 

No one publicly calls Abath a suspect, but federal prosecutors grilled him on these issues last fall. And one former prosecutor in the case has written a recently published novel about the Gardner heist in which the night watchman let the thieves into the museum to pay off a large cocaine debt."

'Abath, then a rock musician moonlighting as a security guard, said he opened the doors that night because he was intimidated by men dressed as police officers who claimed to be investigating a disturbance. His own uniform untucked and wearing a hat, Abath knew he looked more like a suspect than a guard.'

"After years of not hearing a word the people charged with the task of solving the Great Museum Robbery, they popped up; they wanted to talk," Abath wrote in the manuscript he shared. To his surprise, one agent told him, "You know we've never been able to eliminate you as a suspect."