Police say a shelter in place order has been lifted
San Francisco police say an earlier incident at a UPS warehouse in the city has ended.
The courier company say there are "potentially multiple deaths".
UPS confirmed there was a shooting incident involving four employees within its facility.
In a statement, it says: "We cannot provide information as to the identity of persons involved at this time, pending the police investigation.
"We understand that there are potentially multiple deaths, although some individuals were transported to the hospital and we are unsure of their status at this time.
"The company is saddened and deeply concerned about affected employees, family members and the community we share. Our thoughts and prayers are with all those touched by this incident."
The facility is an area package sorting hub and package delivery centre, employing 850 people.
A shelter in place order by police - in the area of 17th Street and Vermont - has also been lifted.
A San Francisco hospital said they received multiple victims from a shooting.
Brent Andrew, spokesman for Zuckerberg San Francisco General Hospital, said that the hospital had received victims, but he did not know exactly how many or their conditions.
Additional reporting: Kenneth Fox