The tape was collected just after the theft, but it is unclear whether it had ever been reviewed before 2013. United States Attorney Carmen M. Ortiz said in a telephone interview that the prosecutor who took over the case about two years ago, Robert Fisher, pulled it from the stacks of Gardner evidence at the F.B.I. and viewed it during a “complete re-examination of the case.”

Since then, she said, Mr. Fisher and F.B.I. investigators have tried to identify the visitor and the make and model of the car. The street images of the car remain dark, and the visitor’s face is only slightly discernible two minutes and 18 seconds into the video.

“Now we are calling on the public for help,” she said.

Asked whether the tape had ever been reviewed 25 years ago, Ms. Ortiz said, “I can’t answer that” and referred the question to the F.B.I.

Pete Kowenhoven, the assistant special agent in charge of the F.B.I.’s Boston bureau, responded: “It may have been reviewed back then. I’m not sure of the answer to that question.” He added that his focus now was on whether the tape would garner new clues.

Ms. Ortiz and Mr. Kowenhoven both said that their agencies would not comment on Mr. Abath “for privacy reasons.”

Anthony M. Amore, director of security at the Gardner since 2005, pointed out that the visit took place right after Mr. Abath’s partner left the guard station to do his rounds. The two fake police officers, who were admitted 24 hours later, after claiming through the intercom that they were looking into a disturbance, also entered the museum just after Mr. Abath’s partner left him to go do rounds, Mr. Amore added.