When, in 1992, a California jury acquitted the four officers who beat Rodney King, the result was a race riot of a kind not seen since the late 1960s — followed by a federal civil-rights prosecution that convicted two of the officers. The acquittal of George Zimmerman for killing Trayvon Martin hasn't produced rioting, but it has spawned a growing demand, led by the National Association for the Advancement of Colored People, for a federal trial to recharge Zimmerman with violating Martin's civil rights.

The laws have changed since 1992 to make bringing such a prosecution easier than it would once have been. Yet the likelihood of getting a conviction would be extremely small. And instead of placating people who feel aggrieved by the first trial, another failed prosecution might make race relations in America worse, not better.

In years past, it would have been difficult, if not impossible, to bring a federal civil-rights charge against a private citizen such as Zimmerman who attacked another citizen on the street. Civil-rights crimes then extended to government officials acting under color of law, or to private citizens who injured other citizens who were engaged in a set of federally protected activities — going to school, registering to vote, etc. The federal laws, in other words, were designed to cover the classic civil-rights situation.

That changed in 2009, when Congress passed the Matthew Shepard and James Byrd Jr. Hate Crimes Prevention Act. The law is best known for making it a crime to injure someone because of sexual orientation. But it began by removing the requirement of a federally protected activity for racially motivated crimes. Under the act, bodily injury is a federal crime if "committed because of the actual or perceived race, color, religion, national origin of any person."

On the surface, this clearly phrased law would seem to make conviction relatively straightforward. All the prosecution would have to do is show that Zimmerman's attack on Martin was racially motivated. Passed pursuant to Congress's authority under the 13th Amendment to remove all incidents and badges of slavery, the law also doesn't require the prosecution to show any awkward link to Congress's power to regulate interstate commerce.

Yet how, exactly, would the federal government prove Zimmerman's motivation? Testimony from a former coworker who claims that Zimmerman made racist, anti-Arab comments on the job would likely not be admissible. Zimmerman's possible racial slur on the tape of his conversation with the 911 operator while following Martin (arguably, "coon") might conceivably be admitted as probative. But a single phrase that is difficult to interpret on the tape is unlikely to be enough to convince a jury that Martin's race was the cause for the attack.

Even if a jury accepted that Zimmerman suspected Martin and pursued him because of his race, the defense attorneys would argue that the actual shooting was motivated by self-defense, not race. And one jury already apparently bought some version of the self-defense story.

It could be argued that the Justice Department should bring federal charges even if the odds of success are slim. The fact of a federal prosecution would show that the government takes hate crimes seriously.

The trouble would arise when another jury — as it probably would — acquitted Zimmerman. Instead of the bottom-line message being that the government wants to protect all its citizens from hate crimes, events might underscore that, in 2013, a white-appearing American can still shoot an unarmed African-American teenager and get away with it not once but twice.

Civil-rights laws are made to be used, and there's no escaping that they have a symbolic value alongside their practical effects. If you believe that Zimmerman killed Martin because of his race, it's easy to understand the appeal of the civil-rights charge. The question for advocates, then, is simple: Would you still support bringing charges even if you believed Zimmerman would get off again?

For the NAACP, the answer may be yes. But for the government, the decision to prosecute must also take account of the odds of success. The consequences of failure here are serious. This time, prudence might counsel letting the coin flip of the Florida jury verdict — however outrageous — stand as the last word.

Noah Feldman is a law professor at Harvard University.