From an old Kausfiles on Slate, a prelude to modern flash mobs:

The Warriors of Rockingham: There was a large gang fight on Halloween in the lush heart of L.A.'s affluent suburban West Side, I'm told. Dozens of teenagers, some wielding bats and chains, from rival black and Hispanic outfits battled each other around 26th Street and San Vicente, on the border between Santa Monica and Brentwood, the latter probably the most expensive neighborhood in Los Angeles. Police were called to the scene in force.** ...Did you learn about this incident in the Los Angeles Times? Of course not. Too interesting! Readers would just want to know more. What's the point? ...[Kausfiles, November 2, 2005]

And an update:

** Update/Correction 11/4: The L.A. Times tells Kevin Roderick's LAObserved  that the police say the Halloween incident described above "didn't happen." I've now reinterviewed my source—an eyewitness who fled the scene for safety reasons—and while I don't have a complete picture I believe there was a gang rumble of some sort, involving dozens of people. The part in the above item that is wrong is the asumption that police were "called to the scene in force." Police were already heavy in the area, as you'd expect on Halloween. But there was apparently no mobilization of additional police or riot cops, no lights flashing, etc. ... I don't know what the police saw and what they didn't see. (My call to Santa Monica police press office hasn't been returned yet). It's hard to believe they'd miss gangbangers walking around with baseball bats. They might have missed fighting in the alleys. But just because the police don't respond doesn't mean nothing happened.  And gangbangers terrorizing trick-or-treaters by fighting each other on Halloween in the heart of Brentwood is a story most Westside parents would want to know about, I should think—even if it's not a story you can get by just calling police headquarters

Yes, but it's a story the LA Times really doesn't want to tell, even if there were police reports.