Double Jeopardy for George Zimmerman?

The Department of Justice wants to waste more money trying him for a federal hate crime.

On Nov. 3, 1979, as members of the Communist Workers Party chanted and sang at a "Death to the Klan" rally in Greensboro, N.C., a group of white men drove up, got out of their cars and began firing at the demonstrators. Five protesters were killed.

Despite video footage, a state criminal trial ended in acquittals for those charged. Amid public outrage at this apparent miscarriage of justice, federal prosecutors indicted six members of the Ku Klux Klan and three members of the American Nazi Party for violating the civil rights of the shooting victims. After a lengthy trial, the defendants were found … not guilty.

The massacre is largely forgotten today, but it was far more shocking than the killing of Trayvon Martin by George Zimmerman, who on Saturday was acquitted of murder and manslaughter. The lesson of the Greensboro episode is that it's really hard to get a conviction of someone who has already been tried and acquitted. That, by the way, is not a bad thing.

The NAACP has gathered a million signatures on a petition urging the Justice Department to file civil rights charges against Zimmerman. He could also be indicted for a federal hate crime. These options are supposed to remedy the failure of the Florida jury to dispense the correct verdict.

The department agreed to consider the suggestion. "Experienced federal prosecutors will determine whether the evidence reveals a prosecutable violation of any of the limited federal criminal civil rights statutes within our jurisdiction," it announced.

You may recall that the Constitution bars trying someone twice for the same offense. The government gets one chance to take away your life or liberty in a criminal trial. A conviction may be appealed; an acquittal is final. If authorities could keep trying until they win, no suspect, however innocent, would ever be safe.

The option of trying someone on a different charge, in a different court system, based on the same conduct, does exist. But it historically has been -- as it should be -- reserved for extreme and outrageous cases. The Zimmerman verdict may have been wrong. But it was not a howling travesty.

When NAACP President Benjamin Jealous compared the case to the notorious 1955 murder of Emmett Till, his example highlighted the weakness of his claim. Till was beaten and shot in the head by white men outraged that he had been disrespectful to a white woman.

An all-white Mississippi jury took barely an hour to acquit the accused men -- and one juror said, "We wouldn't have taken so long if we hadn't stopped to drink pop." The defendants later candidly acknowledged their role, with one of them explaining: "As long as I live and can do anything about it, niggers are gonna stay in their place."

There was never any question, in the Jim Crow South, that Till's murderers would go free. Zimmerman's fate, by contrast, was in doubt from start to finish.

The reason for his acquittal was not that the jurors thought Martin deserved to die. It was a frustrating lack of definitive information about what ensued in the minutes before he was shot. This case was a thick fog of reasonable doubt. For the feds to try to overturn the outcome would be an insult to everyone who took part in the proceedings.

It would also most likely be an exercise in futility. A jury just spent weeks hearing the best case prosecutors could make against Zimmerman and was unconvinced. There is no reason to think the evidence would be more persuasive to another jury.

The federal government did successfully prosecute two Los Angeles police officers for beating Rodney King in 1991, after they got off in a state trial. But that time, there was a tape of the cops using their batons to take batting practice on King's head.

Even so, two other officers were acquitted in the federal trial. An attempt to right the alleged wrong here would almost certainly end the same way, with Zimmerman walking out a free man.

Some critics would like the feds to spend time and money contesting the debatable decision of fallible human beings. But my advice to the Justice Department is to treat state juries according to the old rules for dealing with baseball umpires. Rule 1: The umpire is always right. Rule 2: When the umpire is wrong, see Rule 1.