US Attorney-General William Barr said on Thursday (June 4) that foreign interests and "extremist agitators" affiliated with groups like Antifa have sought to widen divisions in US society following the death of a black man at the hands of a white police officer.
"We've seen evidence that Antifa and other similar extremist groups as well as actors of a variety of different political persuasions have been involving in instigating and participating in violent activity," Barr said at a news conference.
"We're also seeing foreign actors playing all sides," he said.
Barr said federal agents have made 51 arrests so far for charges involving violent activity.
He said 114 law-enforcement officers have been injured in protests in Washington and 22 have been hospitalised.
Authorities are investigating seven fires that were set in violent riots in Washington over the weekend and one person has been arrested and charged, said Regina Lombardo, acting director of the US Bureau of Alcohol, Tobacco, Firearms and Explosives.