A source says the gunman shot himself.
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RELATED: 4 killed, including gunman, in shooting at SF UPS facility
The shooting was reported around 9 a.m. at the packaging and sorting facility at 320 San Bruno Ave.
San Francisco police advised people in other nearby buildings to shelter in place until further notice while they conducted a search of the building.
Police wrote shortly after 10:30 a.m. that the incident has been contained and the building is secure, but investigators are continuing to look through the building for possible victims or witnesses.
Gaut said about 850 employees work at the facility at 320 San Bruno Avenue and they were being evacuated while authorities investigate the shooting.
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The off-ramp from northbound U.S. Highway 101 to Vermont Street had been shut down as a result of the shooting, but reopened shortly after noon. Officials are warning drivers to take alternative routes as there may still be heavy traffic in the area.
Several San Francisco Municipal Railway bus lines that go through the area have also been re-routed.
RELATED: Check out this traffic map to navigate around heavy traffic.
ABC7 News contributed to this story.