WASHINGTON (AFP) - US police said Friday that Michael Brown, the unarmed black teenager whose shooting death by police unleashed riots in a Missouri town, was a suspect in a convenience store robbery.

In a shock turn of events, documents released by Ferguson police linked the 18-year-old student to the theft of a box of cigars shortly before his death in broad daylight on a residential sidewalk.

The force also made public surveillance photos showing a tall, muscular black man - in a T-shirt and khaki shorts like the ones Brown were wearing - shoving a store clerk.

Ferguson police chief Thomas Jackson identified the officer who shot Brown as Darren Wilson, a six-year veteran with no disciplinary record who had been responding to a 911 call about the robbery.

The disclosures came a day after Missouri state troopers took charge of security in Ferguson, displacing heavily-armed local police who had used tear gas and trained sniper rifles on demonstrators.

Jackson's statement prompted renewed, orderly street protests in Ferguson, where the population is almost two-thirds African American but the local police department overwhelmingly white.

In a statement to reporters, the police chief stopped short of linking Brown by name to the "strong-arm" theft of a box of cigars from a convenience store, about 20 minutes before Brown died.

But the connection was made in an incident report, dated Saturday, that summarised the content of a surveillance video taken from the unidentified shop that was allegedly robbed.

"The video reveals Brown enter (sic) the store" with another person, and engaging in "an apparent struggle confrontation" with a store clerk, before exiting with a box of Swisher Sweet cigars.

"It is worth mentioning that this incident is related to another incident (in which) Brown was fatally wounded involving an officer of this department," added the report.

Incredulous local residents suspected an attempt by Ferguson police to shift the narrative, as the FBI and federal Department of Justice pursue a civil rights investigation into the case.

"I think they're covering up a lot of things," said one woman, interviewed on CNN, who did not want her name disclosed.

"They're covering up for this officer... Why wasn't this said in the beginning?"

Witnesses and police have previously given conflicting versions of how the fatal shot was fired.

In the past week, protests escalated into confrontations with police who donned military-style combat uniforms, armed themselves with assault rifles, tear gas and rubber bullets, and deployed armoured vehicles.

The reaction was criticised as heavy-handed and over-militarised by critics from across the political spectrum - from civil rights leaders to Republican Senator Rand Paul to President Barack Obama.

Missouri Governor Jay Nixon reacted on Thursday by standing down the St Louis County police who had led the crackdown, and drafting in the Missouri Highway Patrol, which answers to the state government.

Highway patrol captain Ronald Johnson, who is African-American, was tasked with restoring order. He immediately made his presence felt, turning up in Ferguson without riot gear and talking one-on-one with protesters.

"I grew up here and this is currently my community and my home. It means a lot to me personally that we break this cycle of violence and build trust, and show the utmost respect," Johnson said.

Nixon, however, did not cede to one key demand of the protesters. Control of the investigation into the police shooting of Brown will remain in the hands of the St Louis County force.