Given the contested circumstances of Michael Brown's death, it is understandable that many people were dismayed by a grand jury's rejection of criminal charges against the police officer who shot and killed the unarmed black 18-year-old in Ferguson, Missouri, last summer. But if Darren Wilson had been indicted, he almost certainly would have been acquitted, precisely because important details of his deadly encounter with Brown are hard to pin down.

This much is undisputed: A little after noon on August 9, Wilson saw Brown walking with a 22-year-old friend, Dorian Johnson, in the street and told them to use the sidewalk instead. Johnson said they had almost reached their destination, and Wilson passed them but moments later backed up his police SUV, stopping right next to Brown.

There followed a struggle in which, depending on whose account you believe, Wilson grabbed Brown through the window of the SUV and threatened to shoot him as Brown tried to get away or Brown punched Wilson in the face and tried to grab his gun. Wilson's gun fired twice inside the vehicle, and one of the rounds grazed Brown's hand.

Brown and Johnson took off at that point, and Wilson got out of the SUV. He fired 10 more rounds at Brown, striking him at least six times.

According to some eyewitnesses, Wilson fired at Brown while the teenager was running away, then continued shooting as he turned around and raised his hands in surrender. According to other eyewitnesses, the officer fired as Brown was approaching him in a menacing manner.

Wilson claims he killed Brown in self-defense, fearing the six-foot-four, 300-pound teenager would rush and overpower him. Under Missouri law, the shooting was justified if Wilson reasonably believed it was necessary to prevent Brown from killing or seriously injuring him.

Wilson also could invoke a provision that says police may use lethal force if they reasonably believe it is "immediately necessary" to arrest someone who "has committed or attempted to commit a felony." Wilson says he backed up his car to confront Brown and Johnson after he heard a robbery report on his radio and thought the two young men matched the description of the perpetrators.

Security-camera footage shows Brown stealing a bunch of cigars from a convenience store earlier that day, pushing past a clerk on his way out. That crime qualifies as a felony, and so does Brown's alleged assault on Wilson. Does that mean Wilson was within his rights to fire at Brown as he ran away?

Not quite. As University of Utah law professor Paul Cassell points out, Missouri's remarkably permissive approach to police violence is "patently unconstitutional," since it violates the Fourth Amendment as interpreted by the Supreme Court. In the 1985 case Tennessee v. Garner, the Court said a police officer may use deadly force against a fleeing suspect only when he "poses a threat of serious physical harm, either to the officer or to others."

Four years later, in Graham v. Connor, the Court said the use of force during an arrest is constitutional when it's "objectively reasonable," adding that "the calculus of reasonableness must embody allowance for the fact that police officers are often forced to make split-second judgments—in circumstances that are tense, uncertain, and rapidly evolving—about the amount of force that is necessary in a particular situation." The upshot is that police have extra leeway in using deadly force, although not as much as Missouri's law purports to give them.

Wilson's biggest advantage in a trial would have been one enjoyed by every criminal defendant: The prosecution must prove its case beyond a reasonable doubt. With ambiguous physical evidence, conflicting witness accounts, and no conclusive answers to crucial questions such as who initiated the violence, whether Wilson fired at Brown as he fled, and whether Brown was trying to surrender or trying to attack Wilson, it is hard to imagine how the state could meet that test.

© Copyright 2014 by Creators Syndicate Inc.