The US attorney general is to travel to the riot-hit town of Ferguson, as the federal authorities step up efforts to restore calm after the fatal police shooting of an unarmed black teenager.
Eric Holder will go to the town on Wednesday to meet officials in charge of a federal investigation into the killing of Michael Brown on 9 August.
President Barack Obama urged the local community to "try to unite each other".
Police fired tear gas amid a tense stand-off with protesters.
Footage showed the police arresting several protesters, as lines of armed officers faced up to crowds of protesters.
A photographer working for Getty Images agency was among those arrested, though he was freed after a few hours.
The National Guard has been deployed to support police operations, and Missouri Governor Jay Nixon on Monday lifted a night-time curfew in the town.
Although the town appeared tense and there were localised flare-ups, the protests were generally peaceful.
In a statement, Mr Holder said he would "personally travel" to Ferguson on Wednesday to meet FBI investigators and prosecutors.
"I realise there is tremendous interest in the facts of the incident that led to Michael Brown's death, but I ask for the public patience as we conduct this investigation," Mr Holder said.
And he stressed that the investigation into the shooting of 18-year-old Mr Brown was "a critical step in restoring trust between law enforcement and the community, not just in Ferguson, but beyond".
Separately, President Obama said he understood the "passions and anger" provoked by the death of the teenager.
But he said giving into anger "by looting or carrying guns and even attacking the police only serves to raise tensions and stir chaos".
"It undermines rather than advances justice," he said.
'Objects of fear'
Mr Obama said he recognised that in many communities in the US a "gulf of mistrust" existed between local residents and law enforcement.
"In too many communities, too many young men of colour are left behind and seen only as objects of fear," he said.
But he urged the community to look for understanding, and seek to heal.
Population of Ferguson, Missouri
Black or black African
6% Police officers are black
21% Families living below the poverty line
Earlier, a former New York pathologist hired by Mr Brown's family to perform an independent autopsy into his shooting gave his findings.
Dr Michael Baden said he believed six bullets struck Mr Brown, two of which may have re-entered.
"All of the gunshot wounds could have been survivable, except the one at the top of the head," he said.
Position of surrender
Dr Baden said there were no signs of a struggle, as abrasions around the teenager's face were likely from falling to the pavement after being shot.
He also believed Mr Wilson did not shoot him at close range as there was no gunpowder residue on his body, suggesting the officer was at least 2ft away.
Witnesses have said Mr Brown was shot as he held his hands up in a position of surrender, while the police and supporters of Mr Wilson have said he fired during a fight with Mr Brown.
The officer who shot Mr Brown, Darren Wilson, has been suspended with pay since the shooting.
Mr Brown's family have called for the officer to be arrested and prosecuted.
Another autopsy on Mr Brown will be conducted by the US justice department, in addition to examinations by Dr Baden and St Louis county officials.
Timeline of events
- 9 Aug: Michael Brown is shot by police officer Darren Wilson after an altercation
- 10 Aug: Unrest begins to brew after a candlelight vigil for Mr Brown, with car windows smashed and stores looted
- 11 Aug: FBI opens an investigation into the teenager's killing. Tensions escalate at night, with riot police firing tear gas and rubber bullets at protesters
- 15 Aug: Police release a video purporting to show Mr Brown robbing a convenience store. The move further angers demonstrators and leads to new clashes with police
- 16 Aug: Missouri declares a state of emergency and imposes a curfew in Ferguson
- 17 Aug: Governor calls out National Guard to keep peace
- 18 Aug: Independent post-mortem tests confirm Mr Brown was shot six times, including twice in the head