Four years ago, it was the videotaped police beating of Rodney King. More recently, it was the passage of Proposition 187 in California, a law banning illegal immigrants from receiving non-emergency health care and education. And now it is the scandal surrounding William Masters, part-time actor and screenwriter, and former US Marine.
Last month Mr Masters, who's white, shot and killed an 18-year-old Latino youth while he was on a late-night walk through an otherwise deserted neighbourhood of north LA. Although his victim was unarmed, the Los Angeles District Attorney's Office decided not to prosecute him for any serious offence, prompting an outcry among the city's 3.5m Latinos.
It is an incident that touches on some of rawest nerves in Los Angeles, which is still recovering from the unrest of 1992, the worst riots in modern US history. Mr Masters, 35, encountered two youths spray-painting graffiti on fly-over pillars of the Hollywood Freeway. Like many here, he says he resents the multi-coloured gang-related scrawl that covers mile after mile of the city's freeways, sign posts and buildings; he jotted down the youths' car number, apparently intending to report them to the authorities.
What happened next is in dispute, as there were no independent witnesses. Mr Masters claims that the youths demanded his piece of paper and then tried to rob him, brandishing a screwdriver. As they advanced on him, he pulled out a .38 pistol (for which he had no permit) and shot dead Cesar Rene Arce. The other youth, David Hillo, 20, was hit in the buttocks, receiving minor injuries. Mr Masters says he was in fear of his life; Mr Hillo says he and his friend had a stand-off with Mr Masters, but were retreating.
As soon as the killings became public, Los Angeles' radio talk shows were bombarded with calls, mostly from whites, lauding Mr Masters as a hero and calling for tougher measures against graffiti vandals - known here as "taggers". Some local authorities already offer a $500 (£320) reward to those who inform on tagging crews, creating a modern-day class of West Coast bounty hunters; but there are Californians who feel this is not enough. "The police should be allowed to shoot them on sight," one caller told an interviewer.
Such was Mr Masters' popularity that, during a brief stint in jail after his arrest, an enthusiastic supporter sent him dinner in his cell. Shortly afterwards the District Attorney, Gil Garcetti - who depends heavily on white votes for his election and is already over-stretched by the OJ Simpson trial - announced that Mr Masters would not be prosecuted for murder or manslaughter, as he was acting in self-defence. At most, he may face a misdemeanour charge for not having a weapons permit.
The decision infuriated the victims' relatives and the city's Latino activists. Hispanic lawyers, including the Mexican American Bar Association, sent a delegation to complain to the DA, saying the killing was racially motivated. Last night demonstrators were preparing to gather outside the DA's office for a protest about the Masters affair.
But tensions have worsened as details about the Masters case have emerged. A coroner's examination has revealed that Mr Arce was shot in the back. And Mr Masters - who only spent 41 days training in the Marines before he was discharged, but has a fondness for quoting its jargon - has sounded increasingly like a hot-headed vigilante.
He responded to journalists' questions about the possibility that he may be shot by angry sympathisers of Mr Arce by threatening to "eliminate the enemy", saying they had "better be prepared to die" as he would "take them all to hell" with him. Police records reveal he was once arrested for walking down a street in Austin, Texas, carrying two martial arts swords. He was also arrested, but, he says, not charged, for trying to take a gun into a federal courthouse.
The outrage among Latinos has deepened still further this week. The DA has said that it is considering a prosecution after all. Under California law, it is possible to bring murder charges against someone who takes part in a criminal act that leads to a killing - even if he did not commit it. In other words, if the police shoot a robber, the robber's accomplices can be legally deemed responsible for his death. Prosecutors say they are therefore contemplating bringing charges against David Hillo.