Police have not yet released victims' names but families and friends identified one of the people killed as 46-year-old Mike Lefiti, a UPS driver. A spokesman for the company added they are not releasing the identities of anyone involved at this time.
The shooting happened at around 9:00 a.m.in the area of 17th and Utah.
Police confirmed on Twitter that a shooting had occurred Wednesday morning in the city's Potrero Hill neighborhood. In a military-style lockdown, officers patrolled the area with rifles drawn, evacuating victims and employees.
Assistant Police Chief Toney Chaplin told reporters that the two other people who were shot in the attack, have survived.
The gunman has been shot and was taken to a hospital.
"The suspect put the gun to his head and discharged the weapon", Mr Chaplin said.
- San Francisco Police (@SFPD) June 14, 2017#SFPD contained the incident & building is secure.
Chaplin said police were interviewing hundreds of witnesses. She described the scene as chaotic.
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More than 1 million pounds have been raised to help those affected by the massive blaze which ripped through the building. London Mayor Sadiq Khan faced demands from clearly furious locals for speedy answers and action when he toured the area.
Former congresswoman and gun-safety advocate Gabrielle Giffords, who was gravely wounded in a 2011 assassination attempt in Arizona that claimed six lives, issued a statement lamenting the shootings in Virginia and California, calling them "a stark indication of the scope of gun violence epidemic we face as Americans".
UPS driver Marvin Calderon told KNTV that he recognized the gunman as a fellow employee but did not know him personally. The Special Ops continues to search the building for additional victims & witnesses.
A nearby resident who identified himself as "Chino" told the SF Gate that he saw several people covered in blood being dragged from the building by officers and firefighters and loaded into ambulances.
"I was working and suddenly I heard shots and I peeked out the window", he said. "They were looking pretty bad".
After the shooting, UPS said the company wasn't immediately sure of the status of the survivors who were taken to the hospital.
"I was confused. They crossed the street and when they got to me, they said, 'Run, there is a shooter, ' so I started to run with them", she said.
There has been no words on possible motives of the shooter or what led to the workplace shooting. The gunman fatally shot himself when confronted by police, authorities said. That gunman had recently been fired from the facility. They are routinely surveilled by managers and under constant pressure to meet demanding time and labor requirements.
When the police initially responded to the United Parcel Service facility in San Francisco, Chaplin said they did not know if the suspect was an actual employee there, or if was somebody dressed up like an employee, possibly committing an act of terror. "Any shooting is one shooting too many".