WASHINGTON (AP) — Attorney General Eric Holder said Tuesday he was "disappointed" by the violence in Ferguson, Missouri, that followed a grand jury's decision not to indict Police Officer Darren Wilson for killing 18-year-old Michael Brown.

"It is clear, I think, that acts of violence threaten to drown out those who have legitimate voices, legitimate demonstrators," Holder told reporters. "Those acts of violence cannot and will not be condoned."

Holder said he was encouraged by some of the peaceful demonstrations in the suburban St. Louis city and by those protesters who discouraged others from rioting, calling them "heroes, in my mind."

The attorney general said he has instructed his staff to work with leaders of the nonviolent protests to help root out and isolate anyone bent on destruction. He also has asked for an after-action review to help identify "criminal elements" in otherwise peaceful demonstrations.

Minutes after the decision not to indict Wilson was announced Monday night, protesters poured into the streets of Ferguson. Some commercial buildings were burned down and dozens of people were arrested, many for burglary and trespassing. Missouri Gov. Jay Nixon said more than 2,200 National Guardsmen will be in place in the region near Ferguson on Tuesday night in the event of more violence.

"This is a difficult time for people in Ferguson. It's a difficult time for people in our country," Holder said.

Separate from the county grand jury probe, the Justice Department is conducting a civil rights investigation into the Aug. 9 shooting of Brown, an unarmed black 18-year-old, and is also investigating the policing practices of the entire Ferguson police department.

Holder said the department was looking to complete those investigations "in a timely manner" to restore trust, to rebuild understanding and to foster cooperation between law enforcement and community members."

President Barack Obama, speaking in Chicago, also rebuked protesters for racially charged violence, saying there was no excuse for those actions.

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