ONTARIO (CBSLA.com) — When Brian Sawyer heard a UPS driver shot and killed three colleagues and then himself at the UPS delivery center Wednesday in San Francisco, he wasn’t surprised.

The former security director for UPS at the massive Ontario warehouse, the largest UPS package hub on the West Coast, told CBS2/KCAL9’s Stacey Butler that it was a tough job and that in 2015 he investigated three cases where UPS employees brought guns into work and that he suspects there were many more.

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“The excuses that I heard presented were ‘I forgot. I went to a shooting range and I forgot my gun in my bag,’ ” said Sawyer. “If the employee is caught with a handgun, the union will come to their defense.”

Sawyer said that even though 150 security officers patrol the Ontario facility, they have a big job keeping track of 5,000 employees.

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“Security is not always what it needs to be. It’s very difficult to secure that many employees in a building, especially when they’re coming and going in a free-flowing atmosphere,” explained Sawyer.

Carrying a gun into the UPS center is against company policy. Sawyer said the employees have to pass through a metal detector and bag search to get in and out of the UPS center. But in some cases, drivers found a way to get a gun for their own protection, sometimes for rough areas where they would deliver.

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Sawyer hopes what happened in San Francisco serves as a warning to employees in the workplace to be careful and to report suspicious behavior.