The recent shooting at a San Bernardino, California elementary school illustrates a critical principle about security that applies to every business, school, church, or other workplace: it begins at your perimeter.

Consider the approach used by the professionals who provide the world’s toughest security. When you think of the President’s Secret Service team, your first thought is probably that they are the people who are trained to stand between a shooter and the leader of the free world. But that’s not their primary line of defense.

If a shooter gets close enough that the President requires a human shield, there’s imminent danger. Remember what happened when John Hinckley, Jr. fired upon President Reagan? Even though Reagan was ringed by highly trained and attentive Secret Service agents, one of Hinckley’s bullets found its mark and seriously wounded the President.

The most effective protection provided by the Secret Service is the protection we don’t notice. The first goal is to keep a potential assassin from getting anywhere near the President. So agents establish invisible rings of protection that are essentially fences of people and security tools. Their plan is to stop potential threats at the farthest perimeter from the President.

These days, most schools perform regular lockdown drills, and some have students practice defensive measures they can use in the event someone arrives with a weapon. Many companies and government facilities now provide active shooter training to their employees. Some have panic buttons or secret intercom codes to summon help, and cameras to monitor the movement of people within their buildings. But once an armed subject intent on harming someone is inside the building, the potential for danger skyrockets.

Top corporate security officers will tell you that a domestic violence situation is one of their greatest fears. That appears to be what happened in San Bernardino, where a husband burst into his estranged wife’s classroom, murdered her and then turned his gun on himself.  One student was killed and another was injured in the gunfire. Even if the wife had access to a panic button, if another adult in the room could have dialed 911, or an armed police officer was sitting at a nearby desk, it would have been too late.

The same is true for any of your company, district, or agency’s facilities. If an estranged spouse or someone you terminated storms into your facility with the goal of hurting a family member, a manager, or a co-worker, they’ll likely succeed, no matter what self-defense tools or tactics you have close at hand.

The best way to deal with a potential shooter or anyone else who intends to cause harm to (or even harass) one of your employees is to keep them from getting into your facility in the first place. Security begins with your perimeter, and with knowing who is in your facility and why. You can have an armed guard at your front desk, but if a potential shooter is able to talk his or her way past your security measures, you’ve lost your best opportunity for protection.

A visitor management system is a critical piece of creating that perimeter. It gives you a way of controlling access to your facility and of knowing who is there and why. Investigators are still gathering information about the San Bernardino case, but if the school had a visitor management infrastructure such as our SafeVisitor system and had been aware that the teacher’s husband was a threat, he could have been flagged. While “waiting” for his credentials to be printed, the front office staff could have alerted the police, who would much rather question an individual and prevent a shooting than have to respond after the fact.

If you think a situation like this can’t happen at your location, think again. Domestic abuse reportedly affects one woman in four and one man in 11. And while most people think of it as something that happens at home, it often spills over into the workplace, especially after an abuse victim moves out in an effort to seek safety. The abuser may not know where his victim is living, but knows exactly where to find him or her during the workday.

You owe it to your employees to protect them from threats. Deploying some type of visitor access control is one of the most effective ways to create a perimeter that will minimize the potential of a threat reaching them.

Mike McCarty is CEO of Danville-based Safe Hiring Solutions