It was early morning in March 1990 when two men dressed as police officers buzzed the side door at a Boston museum and claimed they were there to investigate a disturbance.
A little more than an hour later, the men left with what is said to be the most valuable collection of stolen artwork in history: $580 million worth of famous works, including Rembrandt's only seascape, "The Storm on the Sea of Galilee," and Vermeer's "The Concert," a masterpiece valued at more than $200 million.
Investigators over the years have followed leads from Nevada to France, but the priceless items snatched from the Isabella Stewart Gardner Museum have never been recovered.
Law enforcement sources, however, told FoxNews.com that they are confident the crime will be solved and the paintings returned to their frames at the museum -- a four-story Italian-style palace built 110 years ago by Gardner after her husband, a shipping magnate, died and left her a $3 million fortune.
"There's some very viable lines of investigation that we’re working right now," FBI Special Agent Geoff Kelly said in an interview. "This is by no means a cold case. We still continue to get credible leads on a regular basis."
One of those leads could involve Robert Gentile, a 76-year-old reputed Mafia member who pleaded guilty Wednesday in a Connecticut weapons and prescription drugs case. During a hearing earlier this year, a federal prosecutor disclosed that the FBI believes Gentile, of Manchester, Conn., had some involvement with stolen property related to the 1990 heist.
Gentile hasn't been charged in the art heist and insists he knows nothing about it. The theft wasn't mentioned in the plea deal or at Wednesday's hearing. The FBI, meanwhile, will not comment on the Connecticut case, citing a request by the U.S. attorney's office.
The two men who broke into the museum on March 18, 1990 -- hours after Boston celebrated St. Patrick's Day -- had "inside knowledge" of the museum's surveillance system, according to Kelly, who has led the case for the last 10 years.
The suspects, described as white men in their 30s, were disguised as Boston police officers when they approached the museum door. The pair convinced two inexperienced security guards that they were 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 hung just as Mrs. Gardner left them upon her death in 1924.
The pair smashed glass and used box cutters to remove the masterpieces from their frames. In all, 13 priceless items were taken: three paintings by Rembrandt including, "The Storm on the Sea of Galilee," five drawings by Degas, and Vermeer's "The Concert" -- said to be the most valuable stolen painting in the world. 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.
But the method by which the pair seized the works led police to believe they were inexperienced art thieves.
"They were clever in how they got into the museum," said Kelly, "but the working profile points to inexperienced art thieves."
"How they went about removing the paintings – slicing them from their frames – that's indicative of a rank amateur when it comes to art theft," Kelly said. "Anyone who knows anything about art, when you’re taking an old Dutch master, slicing out of the frame will damage the painting."
"My opinion is these guys would have been just as comfortable stealing a car or stealing televisions from people's homes," he said.
The pair also made sure to cover their tracks. They took the museum's surveillance tape with them. They also took a printout from a computer that showed -- based on motion detectors -- where they had walked in the museum. That information, however, was already captured on the computer's hard drive, confirming to authorities where in the museum the thieves had been and how long they had stayed.
"They had a comfort level that really would establish they had some type of knowledge about how the security protocols were conducted at the museum," said Kelly.
Kelly said it's highly probable the thieves had no idea the magnitude of their crime until they woke up the next morning and realized they had committed the "heist of the century." He said it's possible they planned to "wait until the heat dies down" before attempting to sell the works. But it never did.
Over the years, authorities have probed leads all over the world -- from France to Ireland to England. FBI agents in Nevada also investigated a woman's claim that she had seen a Vermeer for sale in a local antique shop. The work turned out to be a replica.
There was nothing distinctive-looking about the suspects, according to Kelly, except that one was wearing a fake mustache. A small Dodge hatchback was seen by witnesses outside of the museum at the time of the crime. Kelly said he believes the car was involved, but noted that it was likely too small to fit the large Rembrandts.
Advancement in DNA testing over the years has allowed authorities to retest evidence at the crime scene. Kelly would neither say what specific items they are examining nor whether they have learned anything from such testing
"It’s possible these things may have changed hands over the years, but I don’t believe this heist was intended to take these and sell them somewhere," he said, noting that the items are too valuable and recognizable to sell on the market.
It's probable they're sitting not far from where they were taken -- in an attic, a basement or behind a wall -- he said.
The museum has a $5 million reward for anyone with information on the whereabouts of the items. The U.S. Attorney in Boston is also offering immunity from prosecution for anyone with information leading to the stolen art.
"I think in this case we’re going to get these paintings back," Kelly said. "Even though these paintings are worth $500 million, really they’re worthless because there’s no market for it. You’d never find a buyer for it."
Anyone with information on the Gardner heist is urged to call the FBI at 617-742-5533.
E-mail this reporter at Cristina.Corbin@FoxNews.com.
The Associated Press contributed to this report.
Cristina Corbin is a Fox News reporter based in New York. Follow her on Twitter @CristinaCorbin.