WASHINGTON (AFP) - The US Justice Department said Wednesday it will not prosecute the Ferguson, Missouri police officer who shot and killed unarmed black teenager Michael Brown last August, touching off nationwide outrage.

Its decision not to prosecute Darren Wilson on federal civil rights charges was issued alongside a report that faulted the police department in the St Louis suburb for systematically targeting African Americans.

"Because Wilson did not act with the requisite criminal intent, it cannot be proven beyond reasonable doubt to a jury that he violated (Brown's civil rights) when he fired his weapon at Brown," the report said.

"For the reasons set forth above, this matter lacks prosecutive merit and should be closed."

Wilson was previously cleared in November by a grand jury in St Louis County, which opted not to indict him for the death of 18-year-old Brown.

The teenager's death - on a residential street in a black-majority town with an overwhelmingly white police force - prompted weeks of sometimes violent protests and kindled a national debate about race relations and law enforcement.

Wilson told the St Louis County grand jury that he had shot Brown - the suspect in the theft of a box of cigarillos from a corner shop - in self-defense.

But others contended that Brown had his hands up in the air when the officer, who has since left the Ferguson police force, opened fire with a handgun.

WIDESPREAD RACIAL BIAS

In its probe into Ferguson's police department, the Justice Department said it found a widespread pattern of racial discrimination and multiple violations of citizens' constitutional rights.

Police officers in the black-majority town of 21,000 regularly pulled over motorists without reasonable suspicion, carried out arrests without probable cause and used excessive force, it reported.

While African Americans make up two-thirds of Ferguson's overall population, they accounted for 93 per cent of all arrests in 2012 through 2014, the report said.

African Americans accounted for 85 per cent of all people stopped by Ferguson police, 90 per cent of all citations issued and 88 per cent of instances in which force was used.

Ferguson's local judicial system also came under a harsh light in the Justice Department report, which said African Americans were 68 per cent less likely than others to have their cases dismissed by a municipal judge.

Investigators also unearthed racial bias in e-mails written by Ferguson police and municipal court officials.

One such e-mail, in November 2008, suggested that President Barack Obama was unlikely to remain in the White House for long because, it said, "what black man holds a steady job for four years."

In a statement on Tuesday, human rights watchdog Amnesty International said the findings should prompt Obama to set up a task force to come up with recommendations for criminal justice reform.

"The United States still has a long way to go before it has truly accountable policing," its US director Steven Hawkins said.

"This country's long history of racial profiling and other police abuses is only matched by its equally long history of inadequate accountability for those responsible."