Southern California in particular has long figured among the nation’s most active regions for extremism, a dynamic driven in part by the region’s rapid demographic change over the past few decades, the lasting influence of white supremacist gangs in California prisons, and a tradition of right-wing radicalism and anti-immigrant sentiment that goes back decades.

In the 1980s, Tom Metzger, a San Diego County resident and former Ku Klux Klan Grand Dragon, who once led vigilante border patrols with David Duke, created White Aryan Resistance, a group which later would be investigated by the F.B.I. for attacks on Jewish organizations and connected with the murder of an African immigrant.

In recent years, San Diego has been the center of gravity for the Western Hammerskins, a chapter of the largest skinhead gang in the United States. In 2012, Wade Michael Page, a member, killed six worshipers at a Sikh temple in Wisconsin.

Identity Evropa, a white supremacist group that has since rebranded itself as the American Identity Movement, was founded by Nathan Damigo, a former Marine who was radicalized after he served a prison term stemming from robbing an Iraqi immigrant cabdriver after a drunken night out in San Diego.

The state’s diversity has increased tolerance among many people who live and work close to others who are different from themselves, but for others it has fueled a sense of alienation and threat, said Lawrence Rosenthal, the chairman of the Center for Right-Wing Studies at the University of California at Berkeley.

“California is obviously the largest state, and it is already majority minority. Which is the nationalists’ and the nativists’ nightmare,” he said.