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 re-charge 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. These included going to school, registering to vote or trying to make use of a public accommodation such as a lunch counter or soda fountain. 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 often called the Matthew Shepard Act for short and is best known for making it a crime to injure someone because of sexual orientation. But Byrd was an African-American murdered by white supremacists in Texas in 1998, and the law actually 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. There is no need to prove that Martin was on a public street or that Zimmerman was trying to deprive him of a civil right. Passed pursuant to Congress' 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' power to regulate interstate commerce.
Yet how, exactly, would the federal government prove Zimmerman's motivation? Testimony from a former co-worker 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 after 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 of 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, with some force, 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. Even with scant chances of success, the fact of prosecution might vindicate federal interest in preserving the right of everyone to walk the streets without being attacked, regardless of race.
The trouble would arise when another jury - as it probably would - acquits 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. One acquittal could perhaps be explained away by the vagaries of the jury system or the particularities of the culture of self-defense as symbolized by Florida's "stand your ground" law.
A second acquittal would mean something different. Because the charge would be that Zimmerman acted with racial animus, the verdict might be perceived as saying that the public didn't believe race was in issue here at all - or worse, that the public didn't care.
The question for advocates then would be simple: Would you still support bringing charges even if you believed Zimmerman would get off again?
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, a law professor at Harvard University and the author of "Cool War: The Future of Global Competition," is a Bloomberg View columnist.