It happened at 9 a.m. Wednesday at the facility on 17th Street in the Potrero Hill neighborhood.
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The ABC7 I-Team confirmed the identity of the shooter as UPS employee Jimmy Lam. An employee said Lam had it out for a manager there and had other problems off the job as well.
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The facility employs 850 people, who evacuated the building as police moved in.
Most UPS workers were told not to speak to reporters following the shooting, but at least one did and he described a chaotic scene.
Driver Marvin Calderon described sitting in his truck waiting for a morning meeting when he heard gunfire behind him and, for a fleeting second, he thought of his family, friends and co-workers before his fight or flight instincts kicked in.
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"It was right behind me. It was like six, seven shots inside and at least five to six inside," Calderon said. "I didn't want to look at the guy."
"I was just scared, just scared for the simple fact that it happened around her. And it's really sad for the victims," said Karla, the sister of a survivor.
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One UPS driver ABC7 News talked to off camera says employees were given the option of going home or going back to work. He chose to go home as many other workers probably did.
There's still a large police presence in the area as the investigation into what happened continues. A lot of roads around Potrero Hill are still blocked off.
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