"You definitely expect to feel safe at work so coming in to somewhere where this happened is quite unnerving. But all we can do is be strong, everyone has to work," said mall employee Shawn Talavera.
San Bruno police said there were two groups of people involved on Tuesday. A person in each group appears to have fired shots at the other group.
No arrests have been made.
Two teenagers were shot. Police said both victims are dealing with serious injuries, but are expected to survive.
Police said on Wednesday evidence at the scene indicated two groups of people were engaged in a dispute on the second floor of the mall when the situation escalated and one person from each group began firing shots at the other.
Authorities said the victims are believed to be associated with the two groups engaged in the dispute.
VIDEO: Employee describes turning off lights, hiding in stockroom during Tanforan shooting
Police also released some video taken from the responding officers' body cameras.
The mall was shut down Tuesday afternoon because of the shootings.
The San Bruno Bart station and parking garage were also closed.
VIDEO: San Bruno police say 2 suspects may have been shooting at each other at Tanforan mall
Police searched trains, stations and the parking garage looking for the shooters. No arrests have been made.
Some commuters said they found themselves looking over their shoulder during their ride Wednesday morning.
"I haven't been thrilled with the security measures so I am always on edge and then something like this happens. It reminds me why I am on edge all the time," said Jon Brown of Walnut Creek.
San Bruno police say they need help from the public on this case. They are asking anyone who was at the mall or knows the people involved to give them a call.
The latest from San Bruno Police, along with pictures from their body cameras. https://t.co/BSIieTAD9p— Amy Hollyfield (@amyhollyfield) July 3, 2019
VIDEO: Victim seen on ground moments after San Bruno shooting