The FBI released a never-before-seen video Thursday that appears to show a security guard at Boston’s Isabella Stewart Gardner Museum allowing a suspicious, unidentified man into the museum the night before a sensational robbery of a half-billion dollars of irreplaceable art.

Federal authorities in Boston released the video Thursday morning as part of a plea to the public for help in cracking what has become the art world’s most important loss and most enduring mystery.

The video shows the suspicious character was allowed to enter the museum through a side door, the same door two thieves disguised as police officers used to bluff their way in the following night. The thieves battered some of the world’s most important art from museum walls and disappeared with it into the night.

Immediately upon its release, the video was refocusing attention on former Gardner security guard Richard Abath. Abath, who appears to be depicted in the video, was an aspiring rock musician a quarter-century ago who supplemented his income as a museum night watchman.

Abath, who now lives in Vermont, is invariably mentioned in discussions of the Gardner heist. He has admitted he violated security policy the night of the robbery when he allowed the two, phony police officers into the museum. Policy required the museum night watchmen to telephone the Boston police, regardless of who was trying to gain entry.

The video footage, captured by museum security cameras 24 hours before the robbery, shows an automobile pull up next to a rear entrance of the museum. The car matches the general description of a vehicle that was reported to have been parked outside the museum moments before the theft on March 18, 1990.

The unidentified man can be seen climbing out of the compact car and being allowed inside by a guard. The guard admitted the man at 12:49 p.m. on March 17, 1990, almost exactly 24 hours before the thieves entered the museum through the same door.

Law enforcement officials would not immediately elaborate on when they discovered the newly released video or their decisions concerning the timing of its release. Boston U.S. Attorney Carmen Ortiz said in a statement Thursday the Gardner thieves took the museum surveillance film recorded the evening of the robbery but overlooked footage from the night before.

The images of the vehicle and the unidentified man are of low resolution, but law enforcement officials hope the release will enable someone among the public to identify the man entering the museum or the car in which he arrived.

Anyone with information regarding the video should call the FBI at 617-742-5533 or the Isabella Gardner Museum at 617-278-5114.

The museum has put up a $5 million reward for information that leads directly to the recovery of all of the stolen items, in good condition. Recovery of one or more of the stolen pieces will result in a partial reward, based upon the value of the piece.

The best lead the Gardner detectives have had in 25 years continues to be Robert “Bobby the Cook” Gentile, a 79-year-old, hitherto unremarkable gangster from Manchester.

Gentile is in a Rhode Island jail awaiting trial following his arrest for selling a loaded handgun to a convicted triple murderer who was working for the FBI. His Hartford lawyer, A. Ryan McGuigan, has complained that the FBI entrapped Gentile on the gun charge to leverage him to cooperate on the museum theft

Gentile, vigorously denies having any involvement in or knowledge of the Gardner job, as does Abath But in Gentile’s case, he has been unable to explain decades of criminal associations and his participation in events and conversations that many longtime investigators say make him the last best hope of cracking the case.

Among other things, the widow of a notorious Boston gangster told the FBI in 2010 that she saw her husband hand Gentile two of the stolen Gardner paintings during a luncheon meeting outside a Portland, Maine, hotel between 2002 and 2004.

“Lies, lies,” Gentile told a federal judge in Hartford. “It’s all lies.”

When Gentile agreed to take submit to a lie detector examination concerning the Gardner heist, the results showed a 99 percent probability he was lying when he denied knowledge or involvement.

Gentile admits acting as a middleman in a confidence scheme built on selling some of the missing Gardner art. But, in repeated interviews, he claims to have been an unwitting participant in the scheme.

Investigators said the thieves talked their way into the museum by claiming to have been dispatched to investigate a disturbance on the grounds. Once inside, they made sure the two guards on duty were unable to reach the panic button that summoned real police officers before subduing them.

The thieves handcuffed both guards and isolated them in different parts of the museum. The panic button was not pushed, the police were not notified and the theft was not reported until hours later, when other museum employees arrived for work.

“Over many months we have engaged in an exhaustive re-examination of the original evidence in this case. Our aim has been to ensure that all avenues have been explored in the continuing quest to recover these artworks,” United States attorney Carmen Ortiz said. “Today we are releasing video images from the night before the theft — images which have not previously been seen by the public — with the hope of identifying an unauthorized visitor to the museum. With the public’s help, we may be able to develop new information that could lead to the recovery of these invaluable works of art.”

The combined value of the 13 works of art stolen during the Gardner theft is at least $500 million, though they are considered priceless within the art community. The following objects were stolen during the burglary and have been missing for the past 25 years: Vermeer’s “The Concert”; Rembrandt’s “A Lady and Gentleman in Black”; Rembrandt’s “The Storm on the Sea of Galilee”; Rembrandt’s “Self Portrait”; Govaert Flinck’s “Landscape with an Obelisk”; A Shang Dynasty Chinese Bronze Beaker from 1200-1100 BC; Degas’ “La Sortie du Pelage”; Degas’ “Cortege Aux Environs de Florence”; Degas’ “Three Mounted Jockeys”; Degas’ “Program for an Artistic Soiree,” charcoal on white paper; Degas’ “Program for an Artistic Soiree,” less finished charcoal on buff paper; Manet’s “Chez Tortoni”; and Napoleonic Eagle Finial.

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