SAN FRANCISCO (CBS SF) – It just seemed like any other day to UPS driver Marvin Calderon as he drove into the company’s massive San Francisco distribution center Wednesday morning.

There would be a short morning meeting. Then he load up his truck and head off to the city’s Presidio neighborhood to deliver his packages.

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“We have a meeting every day, telling us how the day is going to be,” he said of the morning meeting.

Before heading into the meeting, Calderon went back to his truck. Then the shots rang out.

Fellow UPS employee Jimmy Lam had opened fire on his co-workers. By the time he was finished three would be dead and two others wounded.

Lamm would then turn the gun on himself and also be dead.

“I just heard the shots,” Calderon said. “I didn’t see anything because there was a truck in between the gunman and me…There were about 6-7 shots like bang, bang, bang.”

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Calderon quickly scrambled out of his truck and began running.

“I was just running,” he said. “I didn’t wait to look at the gunman…I just started screaming – ‘Get out! Get out!’ It was just gunfire and shots. Everybody stated running…It was too quick. It was less than 30 seconds.”

Calderon said he did not know Lamm, who made his deliveries in the Sunset. He did know one of the victims — Mike Lefiti – a likable co-worker know around the facility as “Big Mike.”

“We called him ‘Big Mike,’” Calderon said. “I don’t know if it is real… I don’t want to believe it… He’s a big guy. A really nice guy…It’s just too much.”

Calderon called the shooting “a life-changing experience.”

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“Your life goes so quick in front of you,” he said. “You think about your kids. You think about your co-workers. The people that you know. My whole family.”