US Attorney General Eric Holder said that while the killing of Trayvon Martin in February 2012 was a "devastating tragedy," there were insufficient grounds to charge gunman George Zimmerman with violating federal civil rights laws.
Holder said in a statement that "a comprehensive investigation found that the high standard for a federal hate crime prosecution cannot be met under the circumstances here."
Martin's killing was the first in a series of high- profile deaths of young black men in recent years, which have sparked a nationwide debate on race in America.
"We, as a nation, must take concrete steps to ensure that such incidents do not occur in the future," he said.
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Zimmerman, 31, a neighbourhood watch volunteer in Sanford, Florida, fatally shot Martin as the 17-year-old unarmed high school student was walking home with iced tea and candy.
He insisted he had been following Martin on suspicion that the youth was involved in robbery, and that he shot him in an act of self-defense.