Sanford, Fla.—When city leaders here feared their community was on the verge of rioting after the killing of an unarmed black teenager, a little-known federal agency quietly parachuted into town to negotiate peace among angry groups.

The killing of Trayvon Martin, shot dead by George Zimmerman who has claimed he acted in self-defense, opened long-festering racial wounds in Sanford. And when black and white religious leaders failed to cool tensions, the stealthy group of so-called “peacemakers” held clandestine meetings and brought together ministers who’d been at odds for decades.

 “I’d hate to say that we couldn’t have done it without them, but I’d much rather learn from someone else’s experience rather than my own misfortune,” Sanford Mayor Jeff Triplett said of efforts by Justice Department office of Community Relations Service.

Zimmerman’s second-degree murder trial began Monday with jury selection. He has said that Martin attacked him the night of the killing and has pleaded not guilty. Community groups, local activists and law enforcement agencies have been meeting and training with CRS representatives in preparation for what is expected to be an emotionally charged trial that could last as long as six weeks.

“There’s no two ways about it. There are going to be people who are going to be unhappy no matter which way it goes and we are preparing for that,” Triplett said.

In the weeks after Martin’s killing on Feb. 26 last year, tensions were at a boiling point as police declined to arrest Zimmerman, citing the state’s Stand Your Ground Laws which give wide discretion in the use of deadly force in self-defense.

As anger over the case grew, Triplett and Norton Bonaparte, Sanford’s city manager, made a trip to Washington, D.C., to meet with Thomas Perez, then the assistant attorney general for the Civil Rights Division.

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