FERGUSON, United States (AFP) - The unarmed black teenager whose shooting death by a white police officer unleashed riots in a Missouri town was a suspect in a robbery just minutes before he was killed, police said on Friday.

The allegation reignited fresh anger and frustration in the St. Louis suburb of Ferguson, with the 18-year-old student's family accusing police of attempting to demean his character amid an ongoing investigation.

Incident reports from the Ferguson police department linked Michael Brown to the theft of a US$49 (S$61) box of cigars from a convenience store, shortly before he was fatally shot by police in broad daylight on a nearby street on Saturday.

Brown's death stirred racial tensions in the majority black suburb with an overwhelmingly white police force and renewed a national debate about relations between law enforcement and African-Americans.

It also called into question the use of heavy-handed policing in response to protesters.

Police released surveillance footage showing a tall, muscular black man - in a T-shirt, khaki shorts and sandals, the same outfit Brown was wearing - grabbing a store clerk by the shirt and shoving him.

Ferguson police chief Thomas Jackson meanwhile identified the officer who shot Brown as Darren Wilson, 28, a white four-year veteran of the force with no record of ill discipline.

At a second press conference on Friday, chief Jackson said the "initial contact between the officer and Mr Brown was not related to the robbery." Rather, he said, Wilson - in a patrol car - had stopped Brown "because he was walking down the street, stopping traffic. That was it." chief Jackson described Wilson as "a distinguished officer" who has been devastated by the shooting. "He never intended for any of this to happen," he added.

- Night of calm -

The disclosures followed Ferguson's first night of calm this week, after Missouri Governor Jay Nixon ordered state troopers to take over from heavily armed and distrusted local police.

"Last night was a great night," said Captain Ron Johnson of the Missouri State Highway Patrol, the African-American officer put in charge of restoring order and who became an instant hero for meeting protesters in person overnight, even shaking hands and hugging some of them.

Protests continued peacefully on Friday, with about 300 people demonstrating at a gas station that has become a focal point since it was looted and torched at the weekend.

The link between Brown and the "strong-arm" theft of a box of Swisher Sweet cigars, about 20 minutes prior to the shooting, appears in a police incident report that summarized the content of a surveillance video.

"The video reveals Brown enter (sic) the store" with his a friend and engaging in "an apparent struggle confrontation" with a store clerk.

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

- 'Devious smear' -

Through their lawyers, Brown's family said they were "beyond outraged" at what they called a "devious" attempt to smear the character of their son, who had no criminal record and was about to start vocational college.

"There is nothing based on the facts that have been placed before us that can justify the execution-style murder of their child by this police officer as he held his hands up, which is the universal sign of surrender," the lawyers added.

Police chief Jackson said he was releasing the incident report and video to the public because he had received "too many requests" from news media under freedom-of-information laws.

But incredulous local residents suspected an attempt by Ferguson police, six days on, to rewrite the narrative - just as St. Louis County police and the FBI conduct parallel investigations.

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

The show of force was denounced as heavy-handed and over-militarized from across the political spectrum, from civil rights leaders to President Barack Obama via Republican Senator Rand Paul.