On the night of April 3, a mother and two children were found stabbed to death in their home near Lithonia. DeKalb County detectives found a knife at the crime scene, and they thought they had the murder weapon. Twelve hours later, a reporter from the Atlanta Journal Constitution spotted a large, bloody knife in plain view on the porch of the home — a potentially key piece of evidence that the DeKalb police completely missed.

Now, veteran prosecutors, police investigators and criminal defense attorneys alike are calling the failure of DeKalb investigators to find the bloody knife incompetent and unacceptable. They’re also saying that the shoddy criminal investigation could derail the prosecution of the man police have accused of the triple murder.

“If they missed something as obvious as a bloody knife then the next question is, what else did they miss? Depending on the rest of the evidence it can have huge consequences,” said veteran prosecutor David Cooke, a former assistant U.S. Attorney and chief senior assistant district attorney for Fulton County and current senior ADA in Houston County.

The police blunder could taint the chain of evidence in the case because it is now unclear exactly where the knife came from.

Police failures to find evidence have affected some criminal cases

“If you have critical evidence found by a private citizen who is not a trained investigator, it could make jurors think the police did not do a good job and maybe have the wrong man,” pointed out one veteran criminal defense lawyer.

The issue of credibility is key. The standard procedure used to search crime scenes is a grid search, in which police systematically comb every square foot of the scene for evidence.

“If it is an important case and you’re going by the grid, you are going to find the knife,” said Cooke, who led Houston County’s Special Victims Unit for four years.

“Even with the best detectives in the world working, something can be missed, something that looks obvious,” countered Danny Agan, who once commanded Atlanta’s homicide unit.

But those mistakes can and should have consequences in criminal cases. One criminal defense lawyer recalled a case in which police claimed an emotionally disturbed man had sprung at them from a closet with what they thought was a gun, so they felt forced to shoot him. Detectives reviewing the incident failed to notice that the bullet pattern showed the man was sitting on the floor when he was shot. The charges against the man were dismissed.

In some cases, evidence found by third parties has been used to support the prosecution, while in others the fact that police missed key evidence has derailed it. It depends on the credibility of who found the evidence and whether there is a clear chain of evidence.

“There is nothing at a crime scene that does not have the potential to be critical,” says Douglas County District Attorney David McDade. “We’ve spent days processing crime scenes because the victims deserve that at the very least.”

As do criminal defendants.

Source: Atlanta Journal Constitution, “Experts: Missed bloody knife could hamper DeKalb prosecution,” Steve Visser and Rhonda Cook, April 11, 2011