Sift Internet forums about the case, and you'll find plenty of those who believe that the heist was an inside job. Though he was tied up during the robbery, nightguard Richard Abath, who openly admitted to sometimes being stoned and drunk on the job, immediately became a person of interest. The inside job theory would account for how the robbers knew where the police emergency button was and where to retrieve the security tapes. Abath also broke protocol by granting the robbers, dressed up as police, entry. Plus, the museum's security system could only detect Abath's footsteps before the robbery.
There has been a recently released surveillance tape of the night before the heist — in it, we see Abath buzzing in a man who he can't recall. Former employees, however, say that the person looked like Abath's boss. Staff, journalists, and investigators do not really seem attached to this theory. It was more likely that Abath or another staff member inadvertently divulged information about the security protocols at the museum to the wrong people.