It should be noted that Mary had recently fired one of the low-wage guys. In the end, taking over two years, local cops went back and tied this one guy to the execution-murder. Originally, he confessed, then he recanted....saying cops put too much pressure on him.
The thing is....not one dime was taken during the episode...while thousands sat there. When they left (believed to be three), they locked the door. This led the cops to only believe that it was an insider.
Generally, cops agree there were three people in the execution, but so far, only one has come to be identified and it would appear the case is closed with the one guy (Carl D Cooper) and he’s doing life in prison.
Over all, it would appear that a massive amount of rage was directed at Ms Mahoney, the former boss who fired Carl.
According to this article it was just a robbery gone bad.