San Francisco has long been a sanctuary for outsiders. Hippies, hobos, queers, and kinksters of all colors have called it home, and it is tempting to believe that hateful extremists could never gain a foothold here....

In the city's Marina District, while starry-eyed newlyweds pose for photos amid the grand columns of the Palace of Fine Arts, a Ku Klux Klansman watches through the second-floor window of his Baker Street home....

And in nearby Pleasant Hill, students play ball in the Diablo Valley College baseball field, unaware that on the other side of a chain-link fence is the suburban headquarters of IslamThreat.com, one of the Bay Area's most notorious anti-Muslim websites.

Hate is hiding all around us, even in the most benign settings. And in the wake of recent hate-fueled shootings this year in Orlando, Fla.; Baton Rouge, La.; and Dallas— and last year's racially motivated church massacre in Charleston, S.C. — the potential dangers posed by Bay Area-based extremists deserve closer scrutiny.