FERGUSON, Mo. (AP) — Missouri Gov. Jay Nixon ordered the National Guard to Ferguson early Monday,

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hours after police used tear gas to clear protesters off the streets following a week of demonstrations against the fatal police shooting of a black Missouri teenager.

In a statement, Nixon said the National Guard would help "in restoring peace and order" to this the St. Louis suburb that has been filled almost nightly with angry, defiant crowds.

"These violent acts are a disservice to the family of Michael Brown and his memory and to the people of this community who yearn for justice to be served and to feel safe in their own homes," Nixon said.

The latest confrontations came on the same day that Attorney General Eric Holder ordered a federal medical examiner to perform another autopsy on a black Missouri teenager who was fatally shot by a white police officer. A preliminary private autopsy found that Brown was shot at least six times, including twice in the head.

As night fell in Ferguson, another peaceful protest quickly deteriorated after marchers pushed toward one end of a street. Police pushed them back by repeatedly firing tear gas, and the streets were empty well before the curfew took effect at midnight.

Authorities said they were responding to reports of gunfire, looting, vandalism and protesters who hurled Molotov cocktails.

"Based on the conditions, I had no alternative but to elevate the level of response," said Capt. Ron Johnson of the Missouri Highway Patrol, who is command in Ferguson.

At least two people wounded in shootings, he said.

The "extraordinary circumstances" surrounding the death of 18-year-old Michael Brown and a request by Brown's family members prompted the Justice Department's decision to conduct a third autopsy, agency spokesman Brian Fallon said in a statement.

The examination was to take place as soon as possible, Fallon said.

The results of a state-performed autopsy would be taken into account along with the federal examination in the Justice Department investigation, Fallon said.

Dr. Michael Baden, a former New York City chief medical examiner, told The New York Times that one of the bullets entered the top of Brown's skull, suggesting that his head was bent forward when he suffered a fatal injury.

Brown was also shot four times in the right arm, and all the bullets were fired into his front, Baden said.

The Justice Department already had deepened its civil rights investigation into the shooting. A day earlier, officials said 40 FBI agents were going door-to-door gathering information in the Ferguson neighborhood where Brown, who was unarmed, was shot to death Aug. 9.

A federally conducted autopsy "more closely focused on entry point of projectiles, defensive wounds and bruises" might help that investigation, said David Weinstein, a former federal prosecutor who supervised the criminal civil rights section of Miami's U.S. attorney's office. The move is "not that unusual," he added.

Federal authorities also want to calm any public fears that no action will be taken on the case, Weinstein said.

Back in Ferguson, Sunday's clashes erupted three hours before the midnight curfew imposed by Gov. Jay Nixon.

Officers in riot gear ordered all the protesters to disperse. Many of the marchers retreated, but a group of about 100 stood defiantly about two blocks away until getting hit by another volley of tear gas.

Protesters laid a line of cinder blocks across the street near the QuikTrip convenience store that was burned down last week. It was an apparent attempt to block police vehicles, but the vehicles easily plowed through. Someone set a nearby trash bin on fire, and the crackle of gunfire could be heard from several blocks away.

Within two hours, most people had been cleared off West Florissant Avenue, one of the community's main thoroughfares. The streets remained quiet as the curfew began. It was to remain in effect until 5 a.m.

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