Two FBI agents were shot and wounded at a house in north St. Louis County early on Wednesday, though the incident was "not directly related" to racially charged unrest in and around nearby Ferguson, an agency spokeswoman said.
One agent was shot in the shoulder and the other was shot in the leg while assisting local police in executing an arrest warrant, FBI spokeswoman Rebecca Wu said in a brief statement. Their injuries were not life-threatening.
"The incident is not directly related to the Ferguson protests," Wu said, but did not elaborate further.
Some 2,000 National Guard troops helped ward off a second night of rampant arson and looting in suburban St. Louis after a grand jury declined to indict a white policeman in the fatal shooting of an unarmed black teenager, and sympathy protests spread to several U.S. cities.
The Wednesday morning incident involving the FBI agents followed hours after a clash between police and protesters in Ferguson.
President Barack Obama appealed for dialog, and his attorney general promised that a federal probe into the Aug. 9 slaying of 18-year-old Michael Brown in Ferguson, Missouri, would be rigorous.
Officer Darren Wilson, the policeman who shot him, said his conscience was clear.
Despite a beefed-up military presence in Ferguson, a police car was torched near City Hall as darkness fell, and police fired smoke bombs and tear gas to scatter protesters. A crowd of demonstrators later converged near police headquarters, scuffled with officers who doused them with pepper spray, then smashed storefront windows as they fled under orders to disperse.
Still, the crowds 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. More than 60 people were arrested then, compared with 44 arrests on Tuesday night, police said.
"Generally, it was a much better night," St. Louis County Police Chief Jon Belmar told reporters early Wednesday, adding there was very little arson or gunfire, and that lawlessness was confined to a relatively small group.
Police and Guard troops mounted a vigorous defense of City Hall out of concern it might come under attack by arsonists, but damage to the building was limited to some shattered windows, Belmar said.
"We saw some protesters out there that were really out there for the right reason," he said. "Unfortunately, there seems to be a few people who are bent on preventing this from happening in the most ideal way that it could."
The unrest surrounding Brown's death in Ferguson, a predominantly black city with a white-dominated power structure, underscored the often-tense nature of U.S. race relations and strained ties between African-American communities and police.
Monday's racially charged protests were more intense than disturbances that followed the shooting itself, though much smaller than widespread rioting and looting that followed the acquittal of police officers in the beating of black motorist Rodney King in Los Angeles two decades ago.
An enlarged contingent of National Guard troops surrounded businesses damaged in Monday's violence. Groups of men also gathered on the roofs of some boarded-up stores to protect the buildings from further damage. Armed with fire extinguishers and, one said, guns, they planned to stay all night.
Elsewhere, protests swelled from Los Angeles to Washington, D.C., on Tuesday.
In New York, police used pepper spray to control the crowd after protesters tried to block the Lincoln Tunnel and Triborough Bridge and marched to Times Square. Several hundred also marched in Harlem, chanting "Racist police!"
Protesters in Los Angeles threw water bottles and other objects at officers outside city police headquarters and later obstructed both sides of a downtown freeway with makeshift roadblocks and debris, authorities said.
In Oakland, California, protesters set rubbish on fire in the middle of a street and swept onto a downtown stretch of Interstate 980, briefly halting traffic. Demonstrators also blocked traffic in Atlanta, where 21 arrests were reported.
Four people were arrested for blocking a roadway in Denver, where police said several hundred people turned out for a protest march. In one of the night's biggest rallies, an estimated 1,500 people took to the streets of Boston, though police there reported just a handful of arrests. Inmates at a correctional facility in Boston taped Brown's name on a window in solidarity with protesters who marched outside.
Missouri Governor Jay Nixon said about 2,200 National Guard troops were to be deployed to the Ferguson area by late Tuesday, more than triple the number from the day before, to help protect homes and businesses and to support local law enforcement.
While no serious injuries were reported, police were investigating the discovery of a body in a car in Ferguson, saying they could not rule out a possible link between the death and Monday night's rioting.
The grand jury decision shifted the legal spotlight to a U.S. Justice Department investigation into whether Wilson violated Brown's civil rights by intentionally using excessive force and whether Ferguson police systematically violate rights through excessive force or discrimination.
Wilson, who could have faced charges ranging from involuntary manslaughter to first-degree murder, told ABC News there was nothing he could have done differently in his confrontation with Brown that would have prevented the teenager's death. "The reason I have a clean conscience is because I know I did my job right," he said, adding he would have acted no differently had Brown been white.
Wilson's lawyer, Jim Towey, later told CNN that his client's life as a police officer was over.
Documents released by prosecutors said that Wilson, who was placed on administrative leave after the shooting, told the grand jury Brown had tried to grab his gun, and that the officer felt his life was in danger when he fired.
"I said, 'Get back or I'm going to shoot you,'" Wilson said, according to the documents. "He immediately grabs my gun and says, 'You are too much of a pussy to shoot me.'"
(Additional reporting by Brendan O'Brien in Milwaukee, Julia Edwards in Washington, Alex Dobuzinskis in Los Angeles, Carey Gillam in Kansas City, David Bailey in Minneapolis, Fiona Ortiz and Mary Wisniewski in Chicago, Jonathan Kaminsky in New Orleans, and Laila Kearney and Letitia Stein in New York, Eric M. Johnson in Seattle.; Writing by Jon Herskovitz and Cynthia Johnston; Editing by Will Dunham, Bernard Orr,Ian Geoghegan, Ralph Boulton and W Simon)
(Additional reporting and writing by Eric M. Johnson from Seattle Editing by W Simon)