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 Justice Department said it will now "evaluate the evidence."

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 who may have 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.