Ferguson decision: The reasons why Darren Wilson was not indicted for shooting unarmed black teenager Michael Brown

A gas station burns as demonstrators protest the Grand Jury decision not to indict police officer Darren Wilson over the shooting death of Michael Brown in Ferguson

Riots have broken out in the city of Ferguson, Missouri after a grand jury decided not to indict a police officer for killing the unarmed black teenager Michael Brown.

Darren Wilson could have faced charges ranging from involuntary manslaughter to first-degree murder, but the jury found there was no probable cause to charge him with any crime following the shooting.

Protesters have expressed outrage at the decision, while Brown’s family have said they were “profoundly disappointed” by the grand jury’s finding.

But how can a police officer shoot dead an unarmed 18-year-old and be found to have acted in keeping with both the law and his training, as Wilson’s lawyers claim?

Leniency towards officers of the law

In declining to indict Wilson, the grand jury reached a conclusion that is far more the norm than the exception.

"For a cop to be indicted and especially to be convicted later of a crime in these kinds of situations is very, very unusual," said Chuck Drago, a police practices consultant and former police chief in Oviedo, Florida.

Wilson’s perception of imminent threat

States and police departments have developed their own policies that generally permit officers to use force when they reasonably fear imminent physical harm. They are also entitled to fire if they believe a suspect likely to cause death or serious injury to another person.

The Supreme Court shaped the U.S. standard in a 1989 decision that said the use of force must be evaluated through the "perspective of a reasonable officer on the scene" rather than being judged after the fact.

Given what's known about Wilson's account of the shooting, it's most likely that the grand jury decided Wilson was justified under self-defence — that he feared for his life when Brown (in his telling) assaulted him in his car and tried to grab at his gun.

Fight with a "felon"

Lawyers for Brown's family say the teen was trying to surrender when he was shot, while Wilson's supporters say the officer feared for his life and opened fire in self-defence.

Protesters march in St Louis, Missouri, to put pressure on the grand jury to charge Officer Darren Wilson over the killing of Michael Brown (AFP)

But results from the autopsy suggest that at least some of the shots fired were when Brown was fleeing.

Whether or not Brown then turned back to “charge” at Wilson again – as it seems is his account – those shots could be justified if Wilson deemed them to be stopping a dangerous felon likely to cause serious harm to someone else.

McCulloch said last night that Brown was a suspect in a nearby robbery – potentially making him a felon.

Inconsistent witness accounts

A number of witnesses have described to the media how they saw Brown standing still with his hands in the air when he was fatally shot.

But McCulloch described a tangled mass of conflicting testimony from 60 witnesses about what happened during the incident that led to Brown's death.

He said that not all of them could be right – and that much of the testimony did not square with the physical evidence.

Autopsy evidence

Witnesses disagreed on whether Brown's hands were up at the time he was shot, McCulloch said, adding that Wilson shot at Brown 12 times. The final shot hit Brown in the top of his head.

The pistol used by Ferguson police officer Darren Wilson to shoot Michael Brown on 9 August (AP)

David Klinger, an associate professor of criminology at the University of Missouri-St Louis and an authority on police shootings, told the Guardian that this fatal last bullet's trajectory suggested a “body tilted forward from the waist”.

“It’s not necessarily evidence that he was charging. But it is evidence that suggests he wasn’t standing still with his hands up.”

Severity of Wilson’s own injuries

Pictures have emerged since the incident that purportedly show the injuries Wilson received.

Though many have pointed out that they don't appear particularly serious, any harm is likely to impact the threat the police officer perceived.

Ferguson police officer Darren Wilson during his medical examination after he fatally shot Michael Brown, in Ferguson

“If an officer is already injured, he is not reasonably going to sit there and let a guy beat him because he can’t use anything but his gun to defend himself,” Klinger said.

The choice to go to a grand jury at all

Some concerns have been raised that the unusual call to hand over the Wilson indictment decision to a grand jury represented a reluctance on prosecutor McCulloch’s part to pursue justice.

Alex Little, a former federal prosecutor, told Vox that McCulloch had already “decided to abdicate his role in the process as an advocate for justice” by giving the decision to a jury.

“At that point, there's no longer a prosecutor in the room guiding the grand jurors, and — more importantly — no state official acting on behalf of the victim, Michael Brown,” he said.

It is worth noting that Wilson could still face action over the shooting.

A separate federal probe into the shooting is still going on, and US Attorney General Eric Holder emphasised that the Justice Department investigators had not reached any conclusions.

For latest coverage of the protests, click here.