It was as if there was a national gasp of disbelief, followed by a roar of anger, when the “not guilty” verdict for Casey Anthony hit the airwaves.

There shouldn’t have been. That verdict is always possible in a trial when direct evidence of guilt is lacking and the prosecution must try to craft a case built around circumstantial evidence — no matter how strong it appears to be to those outside the jury box.

If O.J. Simpson’s trial didn’t teach us that, it should have.

Our society is no stranger to controversial court cases, from Clarence Darrow’s guilty-plea tactic in the thrill-murder trial of Leopold and Loeb in 1924, to the conviction on thin evidence of German-born Bruno Hauptmann in 1935 for the kidnap and murder of the young son of controversial national hero Charles Lindbergh and his wife, the accomplished writer Anne Murrow Lindbergh.

Recently, we have even seen a film based on the trial of those accused of assassinating Abraham Lincoln, with the message that one of the accused was an innocent victim herself.

That’s not to say Casey Anthony may not have borne some responsibility for the death of her daughter, Caylee, or even murdered her. All that anyone but Anthony herself can say for certain, however, is that, in the minds of 12 ordinary Americans — “a jury of her peers” — the state of Florida did not provide sufficient evidence to prove “beyond a reasonable doubt” that she did kill her daughter.

Jurors are legally and morally obligated to insist on such evidence before convicting someone, especially on charges that could draw the death penalty, even if they personally believe the person was “likely” or “probably” guilty. That’s because our justice system isn’t set up to deliver pure justice in every case brought before it.

Instead, it is designed to deal justly with defendants in criminal cases where the penalties can include loss of freedom or even loss of life.

As noted by outside experts, the prosecution tried to build a case when there was no known cause of death, no set time of death, and no evidence of the circumstances surrounding the demise of young Caylee. That prosecutors could not overcome such severe handicaps in the minds of the jurors is not an indictment of the prosecutors or jurors, but of the holes in this case.

Those holes, for better or worse, will let Casey Anthony go free.