One person in each group opened fire, with the two suspected shooters fleeing before officers arrived, according to a statement from the San Bruno Police Department.
The victims, who are both male and in serious condition, were transported to Zuckerberg San Francisco General Hospital, one with a gunshot to the abdomen and the other with one to the leg.
Search teams were deployed to The Shops at Tanforan to search for the suspects and to evacuate the complex. No weapons were recovered at the scene, according to police.
Investigators continue to search for evidence at the mall and are combing through video footage collected in an effort to identify the suspects.
"We are deeply concerned by the shooting that transpired yesterday and are working hard to identify the perpetrators," police Cmdr. Geoff Caldwell said. "To be clear, our number one goal is to identify and arrest the persons responsible for the shooting."