Tensions flared once again after police released surveillance video on Friday that allegedly shows Michael Brown robbing a store minutes before he was shot dead by a police officer. The deadly encounter sparked a week of racially charged protests in Ferguson. (Sarah Parnass/The Washington Post)

Missouri Gov. Jay Nixon on Saturday declared a state of emergency in this roiling St. Louis suburb and imposed an overnight curfew, telling a group of shouting residents that order must be restored after days of protests over the killing of an unarmed black teenager by a white police officer.

The governor’s extraordinary action came as the attorney for a key witness described the shooting of 18-year-old Michael Brown as an execution-style slaying. Lawyer Freeman Bosley Jr. said Dorian Johnson, a friend of Brown’s, has told the FBI that Officer Darren Wilson confronted the two because they were walking in the middle of the street.

Wilson cursed at the pair and ordered them onto the sidewalk, Bosley told The Washington Post. When they refused to comply, he said, the officer grabbed Brown’s throat through the window of his cruiser, pulled out a pistol and shot him. Wilson then chased Brown, shot him in the back and shot him five to six more times as Brown’s hands were raised, Bosley said.

Neither Ferguson police nor the FBI responded to requests for comment on Bosley’s account.

The account, combined with Nixon’s declaration, made for another day of chaos and confusion in this small community, which has been whipsawed for days as police initially put down protests in a paramilitary fashion, then vowed cooperation with demonstrators, and are now cracking down again. The demonstrations have racked Ferguson and ignited a national debate about race and justice in African American communities.

Angry aftermath of the Missouri shooting

With tensions rising, it was unclear how authorities would enforce the midnight-to-5 a.m curfew and whether it would stop the rioting that resumed late Friday, with protesters looting stores and clashing with police wearing riot gear and deploying tear gas.

Capt. Ronald S. Johnson, the Missouri State Highway Patrol commander now in charge of security in Ferguson, said Saturday that the curfew would be enforced through communication, not physical force. “We will be telling people, ‘It’s time to go home,’ ” he said.

As the curfew approached and rain fell, protesters continued to line the streets shouting and chanting, “Hands up, don’t shoot.”

A woman stood in the middle of the main avenue with a sign that said “Pray for Peace.” At the Quiktrip, crowds gathered under the awnings, chanting. Police in riot gear stood quietly in front of the doors of businesses.

Several blocks away, hundreds of police waited in the shopping center parking lot, which has become their staging area.

There were signs earlier that the federal civil rights investigation into Brown’s killing is intensifying. Nixon (D) said it is being “beefed up,’’ with additional FBI agents canvassing the area over the next few days. On Saturday, agents were passing out cards encouraging residents to come forward with information about the shooting.

U.S. Attorney General Eric H. Holder Jr. had criticized the use of force against demonstrators, and it was unclear Saturday how federal officials — who are also monitoring a state investigation of Brown’s death — would react to Nixon’s curfew.

The anger still coursing through the community was visible at Nixon’s news conference Saturday. The governor said he had signed an executive order imposing the extraordinary measures in an effort to find a balance between the protesters’ First Amendment rights and the community’s safety.

Noting that most protesters on Friday night had been peaceful, Nixon said he “cannot allow the ill will of the few to undermine the goodwill of the many, while putting the people and businesses of this community in danger.’’

“This is not to silence the people of Ferguson,’’ Nixon said, “but to address those who are drowning out the voice of the people with their actions. We will not allow a handful of looters to endanger the rest of this community.’’

But after his opening remarks, Nixon quickly lost control of the crowd, with the images being recorded for a national television audience.

“You need to charge that police [officer] with murder!’’ one person yelled. Others demanded to know how the curfew would be enforced. “Going to do tear gas again?” someone asked.

When Nixon began answering that “the best way for us to get peace” was for everyone to go home and get a good night’s sleep, another resident interrupted him, shouting: “We don’t need sleep! We need justice!”

The crackdown was triggered by the wild scene on the streets of Ferguson late Friday night and into Saturday morning. In the main stretch of downtown, which had been tranquil the day before, at least four businesses were looted and reporters were threatened by a small group of rioters. Police deployed tear gas and flash grenades, which dispersed many but seemed to further incite the angriest in the crowd.

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 the store clerk and menacing the person, images that were circulated nationwide and drew a sharp rebuke from Brown’s family.

The video’s release was criticized by the highway patrol and came over the objections of federal authorities, a law enforcement official told CNN on Saturday. The Justice Department had said that distributing the images would heighten tensions in the community, but Ferguson police released it anyway, the official said.

As has been the case the morning after each night of violence, residents hit the streets Saturday morning to help business owners clean up broken glass and other signs of destruction. Business owners and workers were upset both at the looters and at the highway patrol officers. They said officers did not intervene during the looting and had not offered any help afterward with cleanup or investigating the incidents.

“They said they’re not going to do anything,” said Jay Kanzler, the attorney for the Ferguson Market, a liquor and convenience store that was the site of the robbery allegedly committed by Brown. The store was looted Friday night.

Cpl. Juston Wheetley, a spokesman for the highway patrol, did not directly respond to the allegations but said that at least two officers received minor injuries during the rioting.

“There was a lot of looting. Officers got in an area that was trapped. They were pummeled with rocks and bricks in their cars,” Wheetley said. “We feared for their safety. We sent in backup to remove officers from the situation. Backup arrived on the scene and received bricks. That’s when they deployed tear gas.”

On Saturday, hundreds of protesters again marched down West Florissant Avenue, the main strip in Ferguson, shouting “Hands up, don’t shoot!’’

About a quarter-mile away, a crowd gathered at the site of where Brown’s body lay for hours the previous Saturday. Supporters have created a memorial in the middle of the street, with flowers, posters and candles.

About 50 young men in purple shirts that read “Jesus Christ” on the front were lined up nearby. One man in a purple robe was preaching.

Colby Itkowitz in Washington contributed to this report. Markon reported from Washington.