Almost three years to the day since the death of Trayvon Martin, the Department of Justice announced it won’t be bringing federal civil rights charges against his killer, George Zimmerman, who in July 2013 was acquitted in a Florida court of second-degree murder and manslaughter charges. DOJ noted it found “insufficient evidence” to charge Zimmerman under federal law, and Attorney General Eric Holder said in a statement that Martin’s “premature death necessitates that we continue the dialogue and be unafraid of confronting the issues and tensions his passing brought to the surface.”

Holder didn’t mention race, but race was certainly a factor prosecutors considered in determining whether to pursue criminal charges against Zimmerman. DOJ rested its theories on two statutes: One is a '60s-era civil rights law enacted as part of the Fair Housing Act, the other is the Hate Crimes Prevention Act of 2009. When President Barack Obama enacted the latter, he expressed hope that the new law would “strengthen the protections against crimes based on the color of your skin, the faith in your heart, or the place of your birth,” as well as those predicated on “on gender, disability, gender identity, or sexual orientation.”

It’s a big, expansive statute that was more than a decade in the making—even Holder testified in Congress urging its passage. One of its striking features was its source: According to DOJ, it was passed under “Congress’s Thirteenth Amendment authority to eradicate badges and incidents of slavery.” Since its passage, defendants charged under the law have attempted to challenge its constitutionality, arguing race-motivated violence doesn’t amount to “badges and incidents of slavery.” But courts have rejected those claims and upheld the statute. And rightfully so: The point of updating civil rights laws passed in the wake of Reconstruction and the Civil Rights Movement was to make it easier, not harder, to punish crimes of intolerance.

The non-prosecution of Zimmerman should give you some pause. For one, data from the Bureau of Justice Statistics reveals that, in 2012, only one person out of 17 initially investigated under the 2009 law was ultimately convicted. That covers the entire country. Granted, a lot of things can happen from the moment DOJ opens an investigation—plea bargaining, special deals, dropped charges, no charges. But contrast that with the total number of racially motivated incidents reported by state law enforcement agencies to the FBI: 3,297 in 2012. Of those incidents, 66 percent were reported to be motivated by anti-black bias. Though states are free to prosecute those cases under their own laws, there seems to be a disconnect between what’s happening at the state level and what DOJ can do in the most extreme cases.

Then there’s the issue of how difficult it is to prove a federal civil-rights crime. After a thorough review of the evidence, federal investigators concluded they couldn’t prove Zimmerman violated either of the two federal laws they examined. More importantly, they couldn’t prove their case “beyond a reasonable doubt.” In most criminal cases, only a jury decides whether evidence meets that really high standard. But here DOJ took on that role because of the high-profile nature of the case—the department wanted to make sure it had a winning case. And the reasons were more pragmatic than anything else: the potential for a second acquittal in federal court, another media circus, and more suffering for the Martin family were too high a price to pay. Not to mention Holder would’ve opened himself up to criticism with a baseless prosecution.

That explains why DOJ, in its announcement about Zimmerman, went to great lengths to note investigators reviewed every single piece of evidence the Florida case produced—the trial record, forensic evidence, ballistics reports, you name it—and conducted 75 independent interviews plus additional evidence-gathering. Most of that evidence didn’t go to proving Zimmerman’s physical act of injuring or killing Martin—that wasn't in dispute. The biggest hurdle for federal prosecutors was getting inside Zimmerman’s mind, to prove that he “willfully” engaged Martin and took his life because he was black.

Willfulness, in civil rights cases or otherwise, is by far the most difficult thing to prove in criminal law. And absent a damning confession from Zimmerman or a mountain of circumstantial evidence showing that he harbors resentment toward black teenagers, making that showing is hard—so hard, DOJ determined, it couldn’t risk pressing charges and losing later.

If there’s one takeaway from this outcome, which reportedly has the Martin family “heartbroken,” is that at least there’s now an indication of how DOJ will respond in another closely watched case: the civil rights probe of Ferguson officer Darren Wilson. He’s being investigated under a totally different statute, but the legal bar is the same: Will DOJ have enough evidence that Wilson willfully shot and killed Michael Brown because he was black? Don’t hold your breath.