Police in riot gear clashed overnight with small groups of protesters in Ferguson, Missouri amid signs of renewed tensions over the shooting of an unarmed teenager by a local patrol officer.

The flareups, breaking a period of relative calm, came after police released surveillance footage that implicated Michael Brown, 18, in the theft of a packet of cigars from a convenience store a few minutes before he was shot. Brown's family condemned the revelation as a "character assassination" designed to smear the teenager and divert attention from the actions of police.

One of the flashpoints in the early hours of Saturday came at the store where the robbery was alleged to have taken place. Police fired a teargas canister at a crowd of around 200 people who had gathered there. Some protesters threw bottles at officers in riot gear and shouted "hands up, don't shoot", the chant that has come to characterise the protests. According to some accounts, Brown had his hands up when he was shot.

A small crowd later looted the convenience store. A meat market was also looted and other businesses had windows smashed. Police did not intervene.

Other demonstrators and community leaders pleaded with the looters to stop. Claire McCaskill, Missouri's senior US senator, asked outside observers to recognise that the looters represented only a fraction of the protests.

"America, please don't hold small group of looters against hundreds & hundreds of peaceful protesters. Rather hold small group accountable," she said on Twitter.

Governor Jay Nixon was scheduled to meet local leaders on Saturday afternoon, to discuss what his office called "actions to ensure public safety and civil rights". Nixon said earlier on Twitter: "Long night. Thanks to all who tried to stop unnecessary violence"

Antonio French, a local alderman, posted to Twitter pictures of protesters protecting stores from looters.

By Saturday morning the area was calm and largely deserted save for some store owners who braved grey, driving rain – a contrast after a hot, dry week – to guard their stores.

Ferguson police simultaneously released the surveillance footage, a report into the robbery, and named of the police officer who shot Brown as a six-year veteran of the city police department, Darren Wilson. Stoking the tensions, police gave conflicting accounts of whether the events were linked.

Early on Friday, Ferguson police chief Thomas Jackson indicated that the officer stopped Brown in response to a callout over police radio describing the suspect for the robbery. Yet at a second press conference in the afternoon, Jackson said that the "initial contact was not related to the robbery". Brown was stopped because he was walking down the middle of the street, obstructing traffic, the police chief said.

Later still, Jackson was quoted by the St Louis Post-Dispatch as saying that Wilson saw cigars in Brown's hand and realised that he might be a suspect in the robbery.

Attorneys for the Browns said that the family were "beyond outraged" at the simultaneous release, describing it as a strategic move to smear their son.

"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 said in a joint statement.

The FBI, which is investigating whether the shooting breached amounted to a civil rights violation, announced late on Friday that agents would be "canvassing the neighbourhood" over the next few days to find new witnesses.

Jackson said the police report was released in response to freedom of information requests by media outlets, apparently following tip-offs. Such requests can often take weeks or months to be fulfilled.

However, commenting on the resurgence of police clashes with protesters, Captain Ron Johnson, of the Missouri highway patrol, which was handed responsibility for policing the protests on Thursday, admitted he was worried that the release of the information would cause renewed tensions. "We had concerns that this would happen," he said. "I will say we talked all day about the release of the video tape at the food mart."

Johnson, who won plaudits for his handling of the demonstrations after being handed control on Thursday, said he "would have liked to have been consulted" about the simultaneous release of the report of the name and alleged robbery.

"The information could have been put out in a different way," Johnson told reporters and residents at a press conference that unfolded more like a town hall meeting on Friday. "I would have communicated it differently," he told the Guardian afterwards. No details were released about the shooting of Brown, despite that being the subject of similar freedom of information requests.

The disagreement over Friday's decision highlighted the overlapping jurisdictions of the law enforcement agencies involved. The robbery report was released by Ferguson city police, the force that employs Wilson, which has come under sharp criticism for not reflecting the racial makeup of the city, which is majority African American.

The inquiry into the shooting is being led by St Louis County police, which led the policing of the demonstrations over Brown's death through several nights of violent clashes, before being relieved by Missouri governor Jay Nixon on Thursday.

Demonstrators protest on Friday. Photograph: Joshua Lott/AFP/Getty Images

Adding further tension between the various authorities involved, county prosecutor Bob McCulloch, who would be responsible for bringing any charges against the officer, reportedly reacted furiously to Nixon's transfer of the policing of the demonstrations. "It's shameful, what he did," McCulloch told the St Louis Post-Dispatch.

St Louis County police have a warrant for computer hard drives and video from the convenience store involved in the robbery.

More than a dozen officers, including Johnson, the highway patrol captain, visited the store on Friday afternoon and talked with staff.

At the store, customers defended the owner."They're good people. I've known them for over 10 years," said Eugene Ward. "If the evidence shows that the gentleman was here and committed a crime that doesn't change the excessive police reaction. But it also doesn't condone what the young man did."