There was a special on CNN about that robbery. There were only two security guards on the job that night, and it is suspected to be an inside job as the original guard who was on duty that night, called out. So a sub was brought in and another guy, and the robbers somehow appeared to know where the alarm buttons were and tied the two guards up away from them, so they couldn't reach them or touch them. According to one of the guards that were on that night, while he was doing his rounds he noticed like a dark shadow standing between two statues in one of the art rooms, which indicated to him, that they had already had someone inside the building before the robbers even knocked on the door disguised as police.