A grand jury has declined to indict Darren Wilson in the fatal shooting of unarmed black Michael Brown teenager in Ferguson, Mo., prosecutor Bob McCulloch said Monday. (AP)

ST. LOUIS — A St. Louis County grand jury’s decision not to indict Darren Wilson illustrates the difficulty of bringing charges against police officers, particularly when there are such widely divergent eyewitness accounts and because officers tend to be given the benefit of the doubt by jurors, legal experts said.

The grand jury voted to not bring charges against Wilson, the police officer who shot dead 18-year-old Michael Brown three months ago during an encounter on a Ferguson street.

The decision against bringing charges is notable because the legal threshold for a grand jury is lower than at a trial. The grand jurors had to determine only that there was probable cause to believe Wilson committed a crime — not that there was proof beyond a reasonable doubt.

The vote, legal experts say, shows that enough jurors believed that Wilson probably acted in self-defense as he faced off with Brown or that Wilson was within his rights as a police officer to use deadly force.

“If an officer thinks someone is a deadly threat, they are not allowed to fire warning shots or shoot people in the leg,” said Mark Schamel, a criminal defense lawyer who specializes in representing law enforcement officers in use-of-force cases. “If they have a basis to use deadly force, they are trained to shoot center mass until there is no longer a threat. The grand jurors would have been told that.”

Erwin Chemerinsky, a University of California at Irvine law professor, said it is also the job of a grand jury to keep cases from proceeding to trial in criminal courts if they believe there is no chance for a conviction.

The U.S. Supreme Court has repeatedly ruled in favor of police officers, which could have made it difficult to successfully prosecute Wilson. The court has repeatedly said that use of deadly force is justified if officers think they must act to protect the lives of themselves or others or if they believe it is necessary to stop a crime in progress.

“The question for the grand jury is not whether the officer did something wrong,” Chemerinsky said. “The question for the grand jury is a legal one. Could a reasonable jury in this case, after hearing all the evidence, convict the defendant of the crime?”

Platte County Prosecuting Attorney Eric Zahnd, past president of the Missouri Association of Prosecuting Attorneys, said a grand jury in Missouri would be asked to look at state law, rather than at U.S. Supreme Court rulings.

“At the end of the day, they would be looking at state statutes that apply to citizens and police officers,” Zahnd said. “They would be asked to answer a single question: Was the use of force reasonable given the circumstances? No indictment means they believed use of force was reasonable and that there was not probable cause to believe he committed any crimes because of that.”

Civil rights lawyers who specialize in police misconduct cases said the personal biases of jurors also probably played a role. These lawyers said jurors may not have articulated the influence during proceedings or even been conscious of it.

The county has not released jurors’ names — it is prevented from doing so by state law — but it has disclosed the gender and racial makeup. The panelists comprised a black man, two black women, six white men and three white women, court officials said.

Cynthia Heenan, a lawyer with Detroit-based Constitutional Litigation Associates, said citizens, particularly white citizens, want to believe officers.

“Police in America enjoy a special status right now — particularly since 9/11,” Heenan said. “Politicians have decided that the appropriate reaction is to beef up security in every way, from airports to small-town streets. It’s painted this perception that police are the last line between hard-working individuals and criminals. People do not want to believe police can do wrong.”

Complicating the case for jurors was that there were dozens of eyewitness accounts — and many of them were conflicting.

People familiar with evidence presented to the grand jury have told The Washington Post that some eyewitnesses, describing the initial struggle at the police SUV where the first two shots were fired, thought Wilson was the aggressor, while others thought Brown was.

These people said that some eyewitnesses also believed Brown was surrendering at the time Wilson fired the fatal shots, while others said he was not.

National experts on eyewitness testimony said it is not surprising that there would be so many conflicting accounts, particularly of a chaotic crime scene. There are problems with cross-contamination — with some people’s memories influenced by what other people say they saw.

“Memory doesn’t work like a video system. It records in snippets,” said Gary L. Wells, an expert on eyewitness memory and a professor of psychology at Iowa State University. “People tend to fill in missing information and confuse that with the things they actually saw.”

The conflicting accounts probably confused grand jury members, creating doubt — which would work in favor of Wilson, experts said. In the grand jury process, the benefit of the doubt is supposed to given to the defendant, whether the person is a police officer or not, according to Chemerinsky.

“The grand jury exists to protect the potential defendant,” he said. “It’s to make sure that before a person is put through the emotional and financial costs of a trial, the prosecutor has shown enough evidence to a group of citizens to warrant this.”

In the end, Schamel said, the grand jury’s decision to not indict does not mean the jurors believed Wilson is blameless.

“It doesn’t mean he is innocent,” he said. “The grand jury does not explain its findings or comment on the evidence. It just means they did not find probable cause. It doesn’t mean the officer didn’t do anything wrong.”