The police bomb squad was called to 2 World Financial Center in lower Manhattan at midday when a security guard reported a package that seemed suspicious. Brookfield Properties, which runs the property, ordered an evacuation as a precaution.
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Even if the grenade had been real what could it have realistically destroyed? It makes no sense to evacuate an entire building for a threat that encompasses the mail room at most. Everyone in a forty-four floor building was evacuated for 90 minutes in response to a room sized threat.