T’was the night before one of the greatest art heists in history, and all through the Isabella Stewart Gardner Museum, not a creature was stirring -- except for an "unauthorized visitor."

The FBI on Thursday released never-before-seen security footage from March 17, 1990, 24 hours before the infamous Boston art heist. 

The footage shows a security guard letting someone into the museum the day before the theft. It also shows a vehicle parked at the rear entrance that fateful day. 

The vehicle matches the description of the one seen outside the museum moments before the theft. The 25-year-old mystery of two men disguised as police officers who made off with 13 artwork has baffled authorities and art enthusiasts.

Among the  stolen works; Rembrant van Rijn, Edouard Manet, Edgar Degas and Johannes Vermeer.

There is a $5 million reward for any information on leads. No one has been charged with this theft.