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Nixon, who had declared a state of emergency for the town on Saturday and ordered that the streets be cleared for a curfew that ran from midnight to 5 a.m., said the National Guard would fall under the supervision of the Missouri Highway Patrol.
The deployment of the National Guard is the latest in a series of steps taken by authorities to end the looting and burning of stores that have punctuated protests since the shooting death of Michael Brown, 18, more than a week ago.
But retired local business owner Marshall Tucker said: "It ain't getting no better with the National Guard coming in. That'll be worse," he said. "Tonight it's going to get really sticky."
President Barack Obama said he told the governor that use of the National Guard should be limited and urged healing instead of violence. Attorney General Eric Holder will travel to Ferguson on Wednesday, Obama added.
"While I understand the passions and the anger that arise over the death of Michael Brown, giving into that anger by looting or carrying guns, and even attacking the police only serves to raise tensions and stir chaos. It undermines rather than advancing justice," Obama told a news conference.
The president met with Holder earlier on Monday to discuss the Ferguson unrest. The US Department of Justice, the FBI and the St. Louis County Police are investigating the shooting.
An autopsy conducted on behalf of the family of Brown showed he was shot at least six times, including twice in the head. The path of one bullet indicates the 18-year-old may have been lowering his head in surrender when the fatal shot hit, according to Brown family attorney Daryl Parks.
Results of official autopsies by federal authorities and the county are pending.
Parks told a news conference that the family's autopsy showed one bullet hit Brown in the "very top of his head" and another shattered his right eye.
"His head was in a downward position," Parks said. "Given those kind of facts, this officer should have been arrested," Parks said.
There were no signs of struggle with the officer and no gunshot residue on the body. But the lawyers said they had no access yet to clothing, X-rays taken when the county did the first autopsy on Brown's body, or toxicology results, which the county has so far not released.
According to police, the officer involved in the shooting said he fired initially after Brown reached into his police car.
Darren Wilson, 28, the officer who shot Brown as the teenager was walking through a Ferguson residential neighborhood on Aug. 9 with a friend, was put on paid administrative leave and is in hiding. Police say he has been threatened and an online petition is calling for his firing.
Edward Magee, a spokesman for the St. Louis County prosecutor's office, said the case could be presented this week to a grand jury, which will decide whether Wilson will be indicted.
The shooting set off protests in Ferguson, whose population of about 21,000 is mostly black. Thousands of demonstrators, angry that the police officer was not arrested, have filled the streets.
The protests have been marred by rioting and looting, leaving some stores badly damaged, as well as attacks against police with Molotov cocktails, officials said
But law enforcement officials have been widely criticized for using excessive force. Amnesty International USA sent a 13-member human rights delegation to seek meetings with officials in the area and to train local activists in methods of non-violent protest.
U.N. Secretary-General Ban Ki-moon urged US authorities to protect protesters' rights to peaceful assembly and freedom of expression.
According to accounts by local police, Brown and a friend were walking down the middle of the road when Wilson asked them to move onto the sidewalk. Wilson reported that Brown reached into his patrol car and struggled for his service gun when the officer fired the initial shot.
Brown's friend Dorian Johnson, 22, said Wilson had reached out through his car window to grab at Brown and that the teenager was trying to get away when he was shot. Johnson said Brown held up his hands in a sign of surrender but that Wilson got out of his patrol car and shot him several more times.
Police Chief Tom Jackson raised the ire of the Brown family and its supporters for releasing police reports showing that the teen was a suspect in the theft of cigars from a neighborhood convenience mart. The family called that a smear campaign.
Jackson said later the officer did not know Brown was a robbery suspect when he shot Brown and that the incident was tied only to Wilson's request that he move out of the street.