State Governor Jay Nixon lifted an overnight curfew as the soldiers arrived, but tempers were running high amid ongoing controversy over the death of 18-year-old Michael Brown.
A forensic pathologist retained by the victim's family revealed that the student had been shot six times -- twice in the head. Local officials have not released the results of their own autopsy.
In a statement released as Guard soldiers arrived in Ferguson, the majority African-American St Louis suburb at the heart of the unrest, Nixon said the soldiers would come under police command.
Nixon condemned "the firing upon law enforcement officers, the shooting of a civilian, the throwing of Molotov cocktails, looting and a coordinated attempt to overrun the unified command center."
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Brown was shot dead in broad daylight on a residential street on Saturday, August 8 by Darren Wilson, a 28-year-old white police officer.
Store surveillance footage released by local police appears to show Brown shoving an employee after snatching a box of cigars around half-an-hour before he was shot.
Lawyers working for Brown's family today introduced reporters to a respected former New York pathologist who has examined the body and found six gunshot wounds.
"Six bullets struck, and two may have re-entered," Michael Baden said, indicating on a diagram how the bullets hit the top of Brown's cranium, one of his eyes and his right arm and hand.
"All of the gunshot wounds could have been survivable, except the one at the top of the head," he said.
The absence of gunpowder on Brown's body indicated that the muzzle of the gun was probably at least a foot or two -- or as much as 30 feet -- away, Baden added.