The US Department of Justice has cleared Ferguson, Missouri, police officer Darren Wilson of civil rights violations in the August shooting of unarmed black teenager Michael Brown.

Federal officials concluded there was no evidence to disprove Wilson's testimony that he feared for his safety, nor was there reliable evidence that Michael Brown had his hands up when he was shot.

The decision in the August 9 shooting had been expected, in part because of the high legal standard needed for a federal civil rights prosecution.

"Federal statutes require the government to prove that Officer Wilson used unreasonable force when he shot Michael Brown and that he did so wilfully, that is, he shot Brown knowing it was wrong and against the law to do so," the Justice Department said in a statement on Wednesday.

"After a careful and deliberative review of all of the evidence, the department has determined that the evidence does not establish that Darren Wilson violated the applicable federal criminal civil rights statute."

Wilson was cleared in November by a grand jury in the death of Brown — a shooting that touched off protests in the streets and became part of a national conversation about race relations and police departments that patrol minority neighbourhoods.

Wilson told the St. Louis County grand jury that he feared for his life during the confrontation with Brown and that the teen struck him in the face and reached for his gun. Some witnesses have said Brown had his hands up when Wilson shot him.

'Cops violated constitution'

While Wilson was cleared on Wednesday, the Justice Department's report found that the Ferguson Police Department had engaged in conduct violating a number of amendments in the US constitution.

US Attorney General Eric Holder said in a statement that it was "time for Ferguson’s leaders to take immediate, wholesale and structural corrective action" to stop police discrimination from occurring in the city.

“As detailed in our report, this investigation found a community that was deeply polarised, and where deep distrust and hostility often characterised interactions between police and area residents,” he said.

“Our investigation showed that Ferguson police officers routinely violate the Fourth Amendment in stopping people without reasonable suspicion, arresting them without probable cause, and using unreasonable force against them."

Brown's parents Lesley McSpadden and Michael Brown, Sr., released a statement saying they were disappointed Wilson "wouldn't be held accountable for his actions".

"While we are saddened by this decision, we are encouraged that the DOJ will hold the Ferguson Police Department accountable for the pattern of racial bias and profiling they found in their handling of interactions with people of color," their statement said.

"It is our hope that through this action, true change will come not only in Ferguson, but around the country. If that change happens, our son's death will not have been in vain."