Zimmerman was acquitted
of criminal charges in 2013 in the killing of the 17-year-old Martin, who was black.
"Though a comprehensive investigation found that the high standard for a federal hate crime prosecution cannot be met under the circumstances here, this young man's premature death necessitates that we continue the dialogue and be unafraid of confronting the issues and tensions his passing brought to the surface," Attorney General Eric Holder said in a news release. "We, as a nation, must take concrete steps to ensure that such incidents do not occur in the future."
Justice Department prosecutors and officials from the FBI met with Martin's family on Tuesday to inform them of the decision, the department said.
In a Tuesday afternoon statement, the family thanked the Justice Department for its investigation and "millions of people around the world" who supported them.
"Although we are disappointed in these findings, it has steeled our resolve to continue traveling the country with the message of the Trayvon Martin Foundation
, which is dedicated to protecting our youth and empowering those who demand justice and peace," the statement said.
"We remain poised to do everything in our power to help eradicate senseless violence in our communities, because we don't want any other parent to experience the unexplainable loss we have endured. We will never, ever forget what happened to our son, Trayvon, and will honor his memory by working tirelessly to make the world a better place."
The case stirred a groundswell of emotion, much of it centered around race. Civil rights leaders, as well as Martin's relatives, took to the streets contending that the teen -- who'd gone out to get a drink and Skittles from a Sanford, Florida, convenience store only to run into Zimmerman on his way back -- might still be alive today if not for the color of his skin.
Zimmerman, then a neighborhood watch volunteer, never denied shooting Martin. But he argued that he did so in self-defense.
He wasn't charged right away -- his second-degree murder charge didn't come down until April 2012. That was a month after the Justice Department and FBI announced that they had opened an investigation.
Still, the bar was always high to charge Zimmerman under federal hate crime laws, as it is for anyone. The FBI defines a hate crime as "a traditional offense like murder, arson, or vandalism with an added element of bias."
Proving that a person attacked someone is one thing. Proving what was going through their mind when they did it -- especially if the other person is dead -- is much harder.
Holder acknowledged the law's high standard in April 2012: "Something that was reckless, that was negligent does not meet that standard," Holder said. "We have to show that there was specific intent to do the crime with requisite state of mind."