FERGUSON, Mo. -- Attorney General Eric Holder on Sunday ordered a federal medical examiner to perform another autopsy on a black Missouri teenager whose fatal shooting by a white police officer has spurred a week of rancorous and sometimes-violent protests in suburban St. Louis.
The "extraordinary circumstances" surrounding the death of 18-year-old Michael Brown and a request by Brown's family members prompted the order, Department of Justice spokesman Brian Fallon said in a statement.
"This independent examination will take place as soon as possible," Fallon said. "Even after it is complete, Justice Department officials still plan to take the state-performed autopsy into account in the course of their investigation."
As night fell Sunday in Ferguson, another peaceful protest quickly deteriorated after marchers pushed toward one end of a street. Police attempted to push them back by firing tear gas and shouting over a bullhorn that the protest was no longer peaceful.
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 in the street.
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.
President Barack Obama, who has been getting regular updates on the situation in Ferguson while on vacation, was to be briefed by Holder sometime after returning Sunday to the White House.
Back in Ferguson, the latest clashes erupted three hours before the midnight curfew imposed by Gov. Jay Nixon.
Police 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 had laid a line of cinder blocks across the pavement near the QuikTrip convenience store that was burned down last week. It was an apparent attempt to block police vehicles, but the vehicles plowed through with ease. Someone set a nearby trash bin on fire, and gunshots rang out several blocks away.
Earlier in the day, Missouri State Highway Patrol Capt. Ron Johnson said he had met members of Brown's family and the experience "brought tears to my eyes and shame to my heart."
"When this is over," he told the crowd, "I'm going to go in my son's room. My black son, who wears his pants sagging, who wears his hat cocked to the side, got tattoos on his arms, but that's my baby."
Johnson added: "We all need to thank the Browns for Michael. Because Michael's going to make it better for our sons to be better black men."
The Rev. Al Sharpton told the rally Brown's death was a "defining moment for this country."
Sharpton said he wants Congress to stop programs that provide military-style weaponry to police departments. He said he expects police to "smear" the slain teenager, his family and his attorneys. He also condemned the recent violence and looting in Ferguson.
The protests have been going on since Brown's death heightened racial tensions between the predominantly black community and the mostly white Ferguson Police Department, leading to several run-ins between police and protesters and prompting Missouri's governor to put the state highway patrol in charge of security.
Ferguson police waited six days to publicly reveal the name of the officer and documents alleging Brown robbed a convenience store shortly before he was killed. Police Chief Thomas Jackson said the officer did not know Brown was a robbery suspect when he encountered him walking in the street with a friend.
Nixon said Sunday on ABC's "This Week" that he was not aware the police were going to release surveillance video from the store where Brown is alleged to have stolen a $49 box of cigars.
"It's appeared to cast aspersions on a young man that was gunned down in the street. It made emotions raw," Nixon said.
Police have said little about the encounter between Brown and the officer, except to say that it involved a scuffle in which the officer was injured and Brown was shot. Witnesses say the teenager had his hands in the air as the officer fired multiple rounds.
"When you're exhausted, when you're out of resources, when you're out of ammunition, you surrender," Brown's uncle, pastor Charles Ewing, told worshippers during a Sunday sermon at Jennings Mason Temple in Ferguson. "He surrendered and yet he died."
The officer who shot Brown has been identified as Darren Wilson, a six-year police veteran who had no previous complaints against him. Wilson has been on paid administrative leave since the shooting, and the department has refused to say anything about his whereabouts. Associated Press reporters have been unable to contact him at any addresses or phone numbers listed under that name in the St. Louis area.
Also Sunday, about 150 people gathered in St. Louis to show support for Wilson. The crowd protested outside a TV station because it had broadcast from in front of the officer's home.
The St. Louis Post-Dispatch said the station, KSDK, later apologized. Other in the group, composed mostly of police and relatives of officers, carried signs urging people to wait for all the facts.
Death: Tear gas, reports of gunshots reported as police try to disperes crowd ahead of curfew