Saturday night's shooting in the parking lot at Candlestick Park evoked memories of another recent disturbing act of post-game violence involving two rival California pro sports teams - the near-fatal beating this spring of a San Francisco Giants fan outside Dodger Stadium.
In Saturday's attacks, a 24-year-old man, who reportedly was wearing a "F--- the Niners" T-shirt, remained hospitalized in serious condition Sunday after being shot several times in the stomach. He managed to stumble to Candlestick Park stadium security for help despite the severe injuries, police said.
A second victim, a 20-year-old man, was treated for less serious wounds in a separate shooting, also after the game.
Sgt. Mike Andraychak said no arrests have been made and that police are looking for "a person of interest" connected to at least one of the shootings. He would not specify which shooting.
A motive for either attack - including whether either was influenced by emotions surrounding a game involving fiercely rival teams - also wasn't known.
Apart from the shootings, a third victim, a 26-year-old man, was also hospitalized in serious condition Sunday after he was knocked unconscious in a stadium bathroom during the game. That attack appeared unrelated to the other two, police said.
The victims' names have not been released as the violent spree overshadowed the 49ers' 17-3 victory over the Raiders.
The crimes prompted San Francisco Mayor Ed Lee and Oakland Mayor Jean Quan to issue a joint statement saying that violence at stadiums in both cities will not be tolerated.
"The incidents ... are completely unacceptable and will be prosecuted to the fullest extent of the law," the mayors said. "Fans come to our stadiums to enjoy an afternoon of football, not to be subjected to intimidation or violence. These games are family events and the types of images we witnessed last night have no place in our arenas."
NFL spokesman Greg Aiello echoed similar comments, saying "we deplore the activities of a handful of fans at last night's game and pledge our full support to Mayors Lee and Quan and to state and local law enforcement agencies."
49ers coach Jim Harbaugh, who once was a coach in the Raiders organization, said he was saddened to hear about the violence.
"I didn't know anything was going on during the game. I wasn't aware of that," Harbaugh said. "I feel bad for the people who got injured and the people who had to see that, for those who had to witness it."
The team said that "these kinds of events are disquieting to everyone in the Bay Area community. We are working to assist the San Francisco Police Department in any way possible to understand how and why this happened."
Raiders CEO Amy Trask said in a statement that "the incidents at last night's game are not acceptable to the Raiders or to any National Football League team and our thoughts are with all affected."
Head Coach Hue Jackson also shared his desire for a safe fan-friendly environment "where we wish that people come out and enjoy a game and hopefully that those things don't happen."
On Saturday, Sgt. Frank Harrell said the man shot wearing the T-shirt drove his truck to a gate and stumbled to stadium security. A second man shot before that in the parking lot and had superficial face injuries, Harrell said.
He said the two shootings were being treated separately "but we believe they are related."
The attacks come nearly five months after San Francisco Giants fan Bryan Stow was severely beaten by two men in Los Angeles Dodgers gear outside Dodger Stadium after the archrivals' season opener in Los Angeles. Two men charged in the beating, Louie Sanchez, 28, and Marvin Norwood, 30, have pleaded not guilty.
Stow, 42, a Santa Cruz paramedic, suffered severe brain injuries and remains hospitalized in serious condition.
That attack drew widespread attention and focusing the spotlight on security at Dodger Stadium, and the intense rivalry among Dodgers and Giants fans.
Christian End, an assistant professor at Xavier University in Cincinnati, who specializes in sports fan behavior, said there are several factors for unruliness at sporting events - including the magnitude of the game, if it is between arch rivals, adrenaline and alcohol. There's also "deindividuation," when fans supporting a particular team adopt a group mentality and may become uncivil.
"The anonymity of large crowds can afford some fans the opportunity to act in a way that they typically wouldn't because there's less accountability and less fear of repercussion," End said.
End said violence between fans of opposing teams can typically begin with light banter, followed by "one-upping" each other with statistics or other chatter that could draw a crowd.
"Then it could be taken up a notch where the fun aspect is gone and it just escalates," End said.
End said he doesn't believe fan violence has increased in the last 10 years but may appear that way partially due to new technology at hand.
"There are more cameras covering games and more fans using their smartphones," End said. "Any acts of aggression have a higher probability of being captured and being shown over the Internet and on television.
"It would give the impression that, `Boy, fans are engaging in all of this aggressive behavior.' But you have to remember that a vast majority of them are not."
Associated Press sportswriters Janie McCauley in Oakland and Josh Dubow in Alameda contributed to this report.