Never-before-seen video released Thursday shows a security guard admitting an unidentified man into the Isabella Stewart Gardner Museum the night before the infamous 1990 art heist, adding a stunning new clue to Boston’s most enduring mystery.

The video footage, taken by the museum’s surveillance cameras and recently examined by investigators, shows the night watchman open the museum’s side door and grant unauthorized access to the man at about 12:49 a.m. on March 17, 1990 — 24 hours before the museum was robbed by two men dressed as police officers who arrived at the same door.