Missouri Gov. Jay Nixon declared a state of emergency and imposed a curfew Saturday in the St. Louis suburb of Ferguson where black teenager Michael Brown was shot to death by a white police officer a week ago.

Nixon said at a press conference that though many protesters were making themselves heard peacefully, the state would not allow a handful of looters to endanger the community. The curfew will run from midnight to 5 a.m.

Nixon also said Saturday the U.S. Department of Justice is beefing up its investigation of the shooting.

Missouri State Highway Patrol Capt. Ron Johnson, who is in charge of security in Ferguson, said there were 40 FBI agents going door-to-door talking to people who might have seen or have information about the shooting.

Nixon and Johnson spoke at a church in Ferguson, where they were interrupted repeatedly by people demanding justice and objecting to the curfew.

Johnson assured those in attendance that police would communicate with protesters and give them ample opportunity to observe the curfew.

"You saw people sitting in the street and they had the chance to get up," he said. "And that's how it's going to continue."

The latest developments comes after looters were met with little police resistance Friday night and store owners say they were forced to protect their businesses with their own guns, Fox2Now.com reported.

A reporter from the station tweeted that police cars were seen driving past some of the stores being looted and did not respond. Protesters could be seen outside until 6 a.m.

Two store owners, standing outside their business holding guns, told Fox2Now.com that when they called 911, they were sent from one police agency to another, and got no response.

"There's no police," one of the owners said. "We trusted the police to keep it peaceful; they didn't do their job."

Former St. Louis County Police Chief Tim Fitch tweeted: "You did not see "police restraint" overnight. You saw police reluctant to act. We cannot keep stoning the keepers at the gate."

Brown's death had already ignited several days of clashes with furious protesters. Tensions eased Thursday after Nixon turned oversight of the protests over to the Missouri Highway Patrol. Gone were the police in riot gear and armored vehicles, replaced by the new patrol commander who personally walked through the streets with demonstrators. But Friday night marked a resurgence of unrest.

Local officers faced strong criticism earlier in the week for their use of tear gas and rubber bullets against protesters. Johnson said one tear gas canister was deployed Friday night after the group of rioters became unruly.

The officer who killed Brown was identified as 28-year-old Darren Wilson, a six-year police veteran who had no previous complaints filed against him.

The Ferguson Police Department has refused to say anything about Wilson's whereabouts, and Associated Press reporters were unable to contact him at any addresses or phone numbers listed under that name in the St. Louis area.

Wilson has been on paid administrative leave since the shooting. St. Louis County prosecutor Bob McCulloch said it could be weeks before the investigation wraps up.

The Associated Press contributed to this report.