From the article:

During that time, at least two employees X-rayed it in an unsuccessful attempt to determine its contents. One guard shook it in another failed attempt to learn more, and a federal inspector conducted four routine checks of the guard post without discovering that the bag contained an explosive device, according to the report by the inspector general, an independent watchdog.

Yeah, because that’s the first thing you want to do if something might contain explosives—shake it. *headdesk*