National Guard troops and police aimed to head off a third night of violence on Wednesday in Ferguson, Missouri, as more than 400 people have been arrested in the St. Louis suburb and around the United States in civil unrest after a white policeman was cleared in the killing of an unarmed black teenager.

There have been protests in Boston, New York, Los Angeles, Dallas, Atlanta and other cities decrying Monday's grand jury decision not to indict officer Darren Wilson in the Aug. 9 shooting death of 18-year-old Michael Brown in a case that has touched off a debate about race relations in the United States.

U.S. Attorney General Eric Holder told civil rights activists the Justice Department would aggressively investigate police practices in Ferguson, two of the activists said on Wednesday.

Ferguson, a predominately black city, has been hit by two nights of rioting, looting and arson incidents that left several businesses in ashes, but authorities say an increased security presence on Tuesday night helped quell the violence.

Missouri Governor Jay Nixon has deployed about 2,200 National Guard troops in and around Ferguson. Police made 45 arrests in Ferguson in the Tuesday night protests, down from 61 in the aftermath of Monday's grand jury decision.

"The ramped up presence and action of the Missouri National Guard has been helpful," Nixon said on Wednesday after facing criticism for not deploying enough guardsman in the hours after the grand jury's decision.

Tensions between police and black Americans have simmered for decades, with many blacks feeling the U.S. legal system and law enforcement authorities do not treat them fairly. In Washington, President Barack Obama has tried to keep a lid on anger that has spilled over to other cities and garnered international attention.

Obama remained cautious in his comments in the immediate aftermath of the Ferguson shooting, but has been more expansive in recent days including remarks at the White House after the grand jury's decision. On Monday he said deep distrust exists between police and minorities and that "communities of color aren't just making these problems up."

Russia on Wednesday pointed to rioting in Ferguson and the other protests across the United States as evidence that Moscow's detractors in Washington were hypocrites and in no position to lecture Russia on human rights.

St. Louis police said three people were arrested at a protest near City Hall on Wednesday in which activists staged a mock trial of Wilson, who told the grand jury he shot Brown because he feared for his life.

A northern suburb of St. Louis, Ferguson's 21,000 population is mixed, with 63 percent of the residents African-American and 34 percent white, according to 2010 U.S. Census data. Ferguson's mayor, James Knowles, is white, as are most of its city council members.

A 2013 state attorney general's report found more than 85 percent of motorists pulled over in the city are black, and the arrest rate among blacks is twice the rate among white residents.

'Sad fact'

The Justice Department has been investigating whether to bring federal civil rights charges against Wilson and the police department.

"The sad fact is that it brings up issues that we've been struggling with in this country for a long, long time," said Matthew Green, an associate professor of politics at the Catholic University of America.

"These are not problems and issues that are going to get resolved by one president in the remainder of his term."

Wilson told ABC News that there was nothing he could have done differently that would have prevented Brown's death. But the parents of the slain teenager said they did not accept the officer's version of the events.

"I don't believe a word of it," Brown's mother Lesley McSpadden told "CBS This Morning" on Wednesday.

The crowds in Ferguson were smaller and more controlled than on Monday, when about a dozen businesses were torched and others were looted amid rock-throwing and sporadic gunfire from protesters and volleys of tear gas fired by police.

Two FBI agents were shot and wounded at a house in north St. Louis County early on Wednesday and the suspect was dead in an incident not related to the racially charged unrest around nearby Ferguson, officials said.

Protests over the Ferguson decision in several major U.S. cities shut highways on Tuesday night and led to hundreds of arrests. In New York on Wednesday, there were more police than the dozen or so protesters at Union Square, a gathering point for Tuesday's rally in the city.