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Missouri Gov. Jay Nixon on Thursday ordered the state Highway Patrol to take over security in Ferguson, the St. Louis suburb roiled by four nights of unrest over the police killing of an unarmed black teenager.

The governor moved after police came under intense criticism for their handling of the protests, including firing tear gas into crowds Wednesday night and arresting two reporters.

Nixon said that Ferguson in recent days looked "a little more like a war zone, and that's unacceptable."

Highway Patrol officers — wearing no SWAT gear — arrived in Ferguson late Thursday afternoon. Cheers greeted the announcement by the head of the patrol, Capt. Ron Johnson, over a megaphone that he and his officers were "going to march with you."

Johnson told reporters: "I understand the anger and fear the residents of Ferguson are feeling, and our police officers will respect both of them."

Police had said healing racial tension in Ferguson was a priority. The Ferguson Police Department is mostly white, while the city is mostly black. Johnson, who is black, grew up in the community.

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"It means a lot to me personally that we break this cycle of violence," he said.

Nixon said local police would still be involved in patrols, but he said the Highway Patrol would be in charge. He said he hoped the change would "provide a breathing space" to address bigger problems.

"These are deep and existing problems, not only in Missouri, but in America, and this has clearly touched a nerve," he said.

Earlier in the day, President Barack Obama expressed concern about both excessive force by the police and the conduct of the protesters, including looting and vandalism. Attorney General Eric Holder said the "deployment of military equipment and vehicles sends a conflicting message."

Holder spoke with Brown's family by phone Thursday, promising a full, independent civil rights investigation by the Justice Department, a senior Obama administration official told NBC News. The FBI opened that investigation Monday.

The police presence Wednesday night included officers atop armored vehicles, training guns on crowds. Ferguson Police Chief Thomas Jackson said Thursday that crowds had tossed firebombs and bricks at officers, but he acknowledged: "I understand that what it looks like is not good."

IN-DEPTH

SOCIAL

Marching in the street in #Ferguson. Suprise -- the cops are marching with them pic.twitter.com/qpn8YacVUs — Danny Wicentowski (@D_Towski) August 14, 2014

Listening to concerns from community members about need for more respect and peace in #ferguson pic.twitter.com/VXbB9wuSiR — Governor Jay Nixon (@GovJayNixon) August 14, 2014

— Erin McClam, M. Alex Johnson and Pete Williams