Seven decades ago, Latino youths in South Los Angeles banded together to form a gang. They called themselves Florencia, after the east-west thoroughfare that ran through the heart of their territory. Years of violent conflict over that territory with other gangs lent Florencia an identity and reason for being.

Over the years, demographic and social shifts have weakened many street gangs and caused some to die out altogether. Florencia did the opposite, law enforcement officials say, absorbing smaller gangs and expanding their extortion and drug dealing rackets on the orders of its leaders, who are incarcerated many hundreds of miles away.

The gang is now at the center of the killing earlier this month of an off-duty Los Angeles police officer.

Officer Fernando Arroyos, 27, was shot to death the night of Jan. 10 near the intersection of 87th and Beach streets. Police say he was with his girlfriend, looking at a house that he was thinking of buying, when two men robbed him at gunpoint of his wallet and jewelry. Arroyos and his assailants exchanged gunfire. The officer collapsed in an alley, mortally wounded.

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