First and foremost, be certain that all employees, customers, clients, and visitors have been removed from harm's way. Make sure that local law enforcement has been informed of the incident. Provide emergency medical care for any injured parties.
If a threat remains, keep all personnel away from the site until it has been cleared by security or police.
After an incident occurs, it is imperative to reinforce security precautions with all personnel. Frequently, an organization has adequate physical/site security, but the measures have been disregarded or disabled by employees—usually as a corner-cutting convenience.
With the heightened awareness following a critical incident, your personnel will be more likely to hear and heed your warnings against these types of security breaches.