A grand jury in Ferguson, Mo., has decided not to approve criminal charges against white police officer Darren Wilson in the fatal shooting of unarmed black teen Michael Brown.

The decision led to violence and looting Monday night and into Tuesday in the suburb of St. Louis that has gained international attention through weeks of at-times heated and violent demonstrations after Wilson encountered and shot the 18-year-old Brown last August

The grand jury's decision was announced around 9:30 p.m. ET. St. Louis County prosecutor Bob McCulloch ​said the 12-person panel worked "tirelessly to examine and re-examine" testimony from witnesses and evidence.

"Eyewitness accounts must always be challenged and compared against the physical evidence. Many witnesses to the shooting of Michael Brown made statements inconsistent with other statements they gave, and inconsistent with the physical evidence," McCulloch added.

Lesley McSpadden, Michael Brown's mother, wearing sunglasses, reacts as she listens to the announcement of the grand jury's decision Monday night. Brown's family said, 'We are profoundly disappointed.' (Charlie Riedel/Associated Press)

"The duty of the grand jury is to separate fact from fiction."

He cited, as an example, that a number of witnesses claimed they saw Wilson stand over Brown and shoot into his back. Some of those witnesses changed their story "once the autopsy findings were released showing Michael Brown had not sustained any wounds to the back of his body," McCulloch said.

McCulloch said all three autopsies conducted on Brown were consistent, as was the scientific evidence, and the conclusion was that there was no probable cause to lay any charges against Wilson.

McCulloch mentioned that speculation in the media and in social media presented various versions of events, including that Brown had his hands in the air and was standing still when Wilson shot him. But, he said, numerous eyewitnesses whose testimony didn't change had never spoken to the media and their evidence has never been made public.

Some of the Ferguson demonstrations were repelled with firm police force, including firing bean bags into crowds and deploying squads of riot officers in armoured vehicles, a number of whom carried assault rifles while others held non-lethal weapons.

TV footage carried images of burning police cruisers, bricks flying through the air and some looting of shops in Ferguson. Gunshots could be heard going off, but it wasn't immediately clear who was firing at what.

St. Louis police said around 1:30 a.m. local time that about a dozen buildings and two police cars had been torched, but that no loss of life had been reported since the grand jury announcement, despite several gunshots being fired.

The unrest came despite pleas from Brown's family and U.S. President Barack Obama for demonstrations to remain non-violent.

Brown's family released a statement shortly after the decision was announced.

"We are profoundly disappointed that the killer of our child will not face the consequence of his actions," the statement said.

Wilson told grand jury he was struck by Brown

Authorities released more than 1,000 pages of grand jury documents, including Wilson's testimony.

Wilson told jurors that he initially encountered Brown and a friend walking in a street and, when he told them to move to a sidewalk, Brown responded with an expletive.

Wilson then noticed that Brown had a handful of cigars, "and that's when it clicked for me," he said, referring to a radio report minutes earlier of a robbery at a nearby convenience store.

St. Louis County Prosecutor's Office early Tuesday released photos showing Ferguson, Mo., police officer Darren Wilson taken shortly after the Aug. 9 shooting of Michael Brown. They were presented to the grand jury. (St. Louis County Prosecutor's Office/Handout/Reuters)

Wilson said he asked a dispatcher to send additional police, then backed his vehicle up in front of Brown and his friend. As he tried to open the door, Wilson said Brown slammed it back shut.

The officer said he then pushed Brown with the door and Brown hit him in the face. Wilson told grand jurors he was thinking: "What do I do not to get beaten inside my car."

"I drew my gun," Wilson told the grand jury. "I said, "Get back or I'm going to shoot you."

"He immediately grabs my gun and says, "You are too much of a pussy to shoot me," Wilson told grand jurors. He said Brown grabbed the gun with his right hand, twisted it and "digs it into my hip."

Asked why he felt the need to pull his gun, Wilson told grand jurors he was concerned another punch to his face could "knock me out or worse."

Hours before the announcement, and before he himself had learned of the decision, Missouri Gov. Jay Nixon appealed for calm in a televised address.

"Our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect and restraint," Nixon said.

There were reports late in the night that Nixon was calling for more National Guardsmen in the wake of Monday night's violence.

Federal charges not expected

Under Missouri law, the 12-person grand jury only needed nine members to find there is "probable cause" to lay a charge for it to be approved.

The panel had a range of possible charges it could have approved, from first degree murder down to involuntary manslaughter.

The grand jury was convened in mid-August and heard testimony and evidence since then.

A man runs away from the burning storage facility after the announcement of the grand jury decision on Monday. (David Goldman/The Associated Press)

The FBI is conducting a separate federal investigation into whether Wilson breached any of Brown's civil rights. That could result in separate criminal counts, though the Washington Post, citing sources, said investigators have more or less decided federal charges are not warranted.

In his most recent statement on the matter earlier this month, departing U.S. Attorney General Eric Holder said there is no timeline for the FBI to announce the results of its probe.

President Obama delivered a televised address Monday night, saying the distressed reaction to the grand jury decision is "understandable" but nevertheless "hurting others or destroying property is not the answer."

"The fact is, in too many parts of this country, a deep distrust exists between law enforcement and communities of colour," Obama said.

"The good news is there are things we know we can do to help​."

It's not uncommon for deliberations to take a while in complex cases when, such as in the Brown shooting, self-defence is alleged or there are two widely conflicting versions of events, said Cole County prosecutor Mark Richardson, who is not involved in the Ferguson case.

St. Louis County prosecutor Robert McCulloch said 'the physical and scientific evidence... tells the accurate story,' whereas some witnesses acknowledged they didn't actually see the shooting or heard about it from others. (Cristina Fletes-Boutte/Pool/Getty)

"In the course of their deliberations, if one grand juror convinces the others that 'Look, we need to hear from an additional witness,' and they all agree, the prosecutor's got a duty to bring that witness in," Richardson said.

Throughout the investigation, some black leaders and Brown's parents questioned McCulloch's ability to be impartial. The prosecutor's father, mother, brother, uncle and cousin all worked for the St. Louis Police Department, and his father was killed while responding to a call involving a black suspect in 1964.

McCulloch was 12 at the time, and the killing became a hallmark of his initial campaign for elected prosecutor.

A Democrat, McCulloch has been in office since 1991 and was re-elected to another term earlier this month.