The federal government on Sunday took on an intensified role in investigating the fatal police shooting of an unarmed black teenager, but violent unrest erupted once again Sunday night on the streets of this St. Louis suburb.

Attorney General Eric H. Holder Jr. issued a terse statement ordering an independent autopsy of Michael Brown by a federal medical examiner — the third autopsy of the 18-year-old. The Justice Department took the action in response to what spokesman Brian Fallon called “the extraordinary circumstances involved in this case” and requests from Brown’s family. Brown was killed Aug. 9 by a white police officer, Darren Wilson.

On Sunday evening, crowds angered by the incident and its aftermath gathered again. Police said protesters fired at least 30 gunshots and threw Molotov cocktails at officers. Police blocked off Ferguson’s main avenue, and hundreds of officers in riot gear advanced on the crowd and began firing tear gas. Some of the demonstrators, covering their faces with scarves and napkins, tossed the tear gas canisters back at police, who included officers from the city of St. Louis, the county and the State Highway Patrol. Protesters began looting Papa John’s and Imo’s pizzerias.

The air was thick with smoke, and people could be heard shouting, “Hard to breathe!”

Natasha Cornell, 42, who lives two blocks from the scene, said she was furious.

“Half of these protesters are not even from this area,” she said. “We don’t have too many stores to go to anymore because they burned it all up. My 20-year-old son ran all the way home from work. He was terrified. It’s not a race issue at this point, people just want to vent and loot.”

By 10:30 p.m., about 20 ambulances were standing by and helicopters were circling overhead. Police were pointing guns at protesters, who were running and screaming, “Help! Help!” Police said that at least one person was injured.

A police officer said: “It’s so unsafe here. People are shooting at each other, bullets are flying, and police are on high alert.”

Richard Eckhard, a spokesman for the St. Louis County police, said: “It’s an active scene. A fluid scene. Obviously, from what you are seeing, we are on high alert and information is still coming.”

An employee at a McDonald’s restaurant near the scene said: “People went crazy. They were trying to get into the McDonald’s because they were tear-gassed. It was some crazy stuff at the restaurant.”

About 11:15 p.m., West Florissant Avenue, the street where Brown took some of his final steps, began to clear.

Sunday morning, Missouri Gov. Jay Nixon (D), following a chaotic night marked by gun violence, stood behind his decision to order a ­midnight-to-5 a.m. curfew, saying on CNN’s “State of the Union”: “Last night’s curfew — I think everybody worked well. We’re always disappointed when things aren’t perfect. But thousands of people spoke last night, thousands of people marched, and [there was] not a single gunshot fired by a member of law enforcement last night.”

Angry aftermath of the Missouri shooting

Capt. Ronald Johnson of the Missouri Highway Patrol, who is overseeing security in Ferguson and had at one point mingled with protesters, took a new tack on Sunday afternoon, calling the police response “proper” and saying he was “disappointed” in the actions of the demonstrators.

At a rally Sunday at Greater Grace Church in Ferguson, Michael Brown Sr. wore a T-shirt that said “No Justice No Peace.” It had a photo of his son as a baby on the front. The church was packed to overflowing, and cars were lined up outside for more than a mile.

Benjamin Crump, the attorney representing the family, said that Michael Brown’s parents did not want to speak.

“It is just a heavy heart they’ve been dealing with,” he said. “Nobody volunteers this position. Nobody wakes up and says, ‘I want to be the mother of a child executed in broad daylight.’ No matter how they try to distract us, we know there was an execution.”

A man who identified himself as a family member said that Brown “was a son and uncle and nephew. He was not alone, but that is how he was killed. This was the last act our family member made before he was put to rest.

“This,” he added, putting his hands up in the air, “this will be stuck in our family’s memories for all our lives.”

The Rev. Al Sharpton, who attended the rally, said the family is calling for the federal government to take over the case, “because if you look at what the police chief did, how can you trust the local authorities?”

He told the crowd that the investigation will be a defining moment in the country’s justice system.

“In all my life, I’ve never seen anything more despicable than the police chief releasing a tape to disparage Michael Brown’s name while his mother is still weeping,” Sharpton said, referring to a convenience store security tape that police said showed Brown stealing cigars.

“This young man hadn’t even been buried. Then they come out and say [the incident] had nothing to do with the shooting. Then why did you put it out? I saw the tape of a young man who might be shoplifting. There’s a difference between robbing and shoplifting. This issue is not whether he shoplifted. The issue is about a young man with no due process who was shot multiple times.”

About 50 white demonstrators gathered Sunday night in downtown St. Louis near Busch Stadium wearing blue shirts, with police emblems, that said “I stand by you, Darren Wilson.”

Some said they were friends of the officer. They held signs that said, “Honk if you support Wilson.”

Some drivers, including some African Americans, honked. Others booed or raised a middle finger.

Todd Brayfield, who works in pipe repair, said he came out to support all police officers.

“We need to support police,” he said. “We don’t know the whole story, and we shouldn’t call [Wilson] a murderer.”

Brayfield said he is upset about the rioting. But he added: “I’m not on that side [of town], so I don’t know what it’s like. But I know we don’t have all the facts yet.”

Three female police officers on bicycles stood by during the gathering, which remained peaceful and lasted just over an hour.

In Washington, the White House said President Obama was briefed Sunday morning on the events in Ferguson and was scheduled to receive another briefing Monday morning from the attorney general.

The federal investigation of Brown’s death will take into account the autopsy performed by state medical examiners, in addition to the procedure ordered by Holder, Justice Department officials said. According to federal law enforcement officials, Brown’s family also paid for an autopsy by Michael Baden, a medical examiner who was featured in the HBO show “Autopsy.” Baden has consulted on investigations relating to the deaths of civil rights leader Medgar Evers and actor John Belushi.

Baden’s autopsy found that Brown was shot at least six times — twice in the head — and that all of the bullets struck him in the front, the New York Times reported late Sunday. The bullets did not appear to have been fired from close range because no gunpowder was found on Brown’s body, the newspaper reported. Dorian Johnson, a friend who was with Brown and ran from the scene during the shooting, has said through an attorney that Brown was shot in the back.

Holder’s autopsy announcement was the latest signal that the federal civil rights investigation of Brown’s death is escalating. Dozens of FBI agents are going door to door in Ferguson to interview anybody with information about the shooting.

The federal probe is running parallel to the state investigation, and federal officials said last week that they were deferring, for now, to state officials.

The new developments came after a Saturday night in Ferguson that ended with a shooting victim, seven arrests and a heavy early morning rain that finally helped clear the streets.

Johnson said early Sunday that a large force was deployed amid the curfew and protests after police received a report that an unknown assailant had shot a person.

Officers had learned through intelligence sources that a group of armed protesters was holed up in a barbecue restaurant. He said that by the time police arrived, the shooting victim, whom Johnson did not identify, had been taken to a hospital by protesters. The victim was in critical condition, he said.

Tear gas was fired, he said, after officers spotted a man with a handgun in the middle of the street. The man with the gun fled, and officers did not arrest any armed protesters. However, seven people were arrested and charged with “failure to disperse,” he said.

A shot was fired at a police car, Johnson said, though it was unclear whether the vehicle was hit.

When the five-hour curfew ended Sunday at 5 a.m., the streets were quiet. But less than an hour after the curfew had begun, police were battling protesters in the streets where Brown was killed.

Hundreds of protesters stood in the middle of Ferguson’s main avenue under heavy rain early Sunday, minutes after the curfew went into effect at midnight. The crowd chanted, “No justice, no curfew!” and “Hands up, don’t shoot!”

At 12:41 a.m., police shouted over a loudspeaker: “You are violating a state-imposed curfew. You must disperse or you will be subject to arrest or other actions.”

Some in the crowd left. Others shouted expletives at the police.

Then came disorder.

At 12:49 a.m., police fired tear gas canisters and devices that produced smoke. Protesters ran. Some were handcuffed. Shots were fired. Police sirens wailed. By 1:30 a.m., a plume of smoke rose over West Florissant Avenue. The smell of smoke was in the air. Explosions erupted every 10 minutes or so — more canisters that made loud bangs.

Police in riot gear blocked the entrance to the main road. They held shields, pointed rifles and shouted for people to clear the road. Many dispersed. By 2:45 a.m., the police had turned much of Ferguson into a ghost town. A heavy downpour puddled on streets emptied of inhabitants. Two officers ran down West Florissant, shedding gas masks without breaking stride. The flashing lights of dozens of police vehicles reflected off the rain-slicked pavement.

“This is not our community!” an onlooker said. She made a peace sign with her right hand, then talked of “revolution.” She wouldn’t give her name.

The confrontation early Sunday followed Nixon’s declaration on Saturday of a state of emergency and the overnight curfew. In a heated news conference, the governor told a group of shouting residents that order must be restored after days of protests.

Yet the renewed protests were apparently triggered by the actions of the authorities, who have been wrestling for days with how to balance public safety with the right of demonstrators to assemble. On Friday, Ferguson police had named Brown as the prime suspect in the robbery of a convenience store that occurred just before the shooting, and they released a video of the robbery. The footage showed someone they identified as Brown towering over and menacing a store clerk. The images were circulated nationwide and drew a sharp rebuke from Brown’s family.

Sarah Larimer, Monica Hesse, Sari Horwitz, Adam Goldman, Alice Crites, Sean Sullivan and Kimberly Kindy in Washington and Katie Zezima in Oak Bluffs, Mass., contributed to this report. Markon reported from Washington.