George Zimmerman, the former neighbourhood watch volunteer who fatally shot Trayvon Martin in a 2012 confrontation with the teenager, will not face US federal charges.
The Justice Department's decision resolves a case that focused public attention on self-defence gun laws and became a flashpoint in the national conversation about race two years before the Ferguson, Missouri, police shooting of a black man.
Zimmerman has maintained that he acted in self defence when he shot the 17-year-old Martin during a confrontation inside a gated community in Sanford in Florida.
Martin, who was black, was unarmed when he was killed. Zimmerman identifies himself as Hispanic.
Once Zimmerman was acquitted of second-degree murder by a state jury in July 2013, Martin's family turned to the federal investigation in final hopes that he would be held accountable for the shooting.
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That probe focused on whether the killing could be charged as a federal hate crime and on whether Zimmerman wilfully deprived Martin of his civil rights, a difficult legal standard to meet.
But Justice Department officials said they ultimately determined there was insufficient evidence to prove Zimmerman killed the teenager on account of his race.
"Our decision not to pursue federal charges does not condone the shooting that resulted in the death of Trayvon Martin and is based solely on the high legal standard applicable to these cases," Vanita Gupta, the Justice Department's top civil rights official, said in a statement announcing the decision.
Martin's parents were too distraught after their meeting in Miami with Justice Department officials to speak with reporters, said their lawyer Ben Crump, who called the decision a "bitter pill to swallow" even though it was expected.
"What they told his family and I was that because Trayvon wasn't able to tell us his version of events, there was a lack of evidence to bring the charges. That's the tragedy," Crump said.
The February 2012 confrontation began after Zimmerman observed Martin while driving in his neighbourhood.
Zimmerman called police and got out of his car and approached Martin, who was returning from a store while visiting his father and his father's fiancee at the same townhouse complex where Zimmerman lived.
Zimmerman did not testify at his trial, but he told investigators that he feared for his life as Martin straddled him and punched him during the fight.
The decision to not prosecute Zimmerman comes even though Attorney-General Eric Holder has made civil rights a cornerstone of his tenure.
The Justice Department is moving to resolve a separate high-profile civil rights case: the August shooting by a Ferguson police officer of Michael Brown, an unarmed black 18-year-old.
Holder has indicated that he plans to announce a decision in that case, which prompted weeks of protests, before he leaves the Justice Department in the coming weeks.
In the Ferguson case, the Justice Department has been investigating whether the officer deprived Brown of his civil rights by using excessive force.
