When the F.B.I. has investigated neo-Nazi organizations like the Base and Atomwaffen Division, it has treated them as criminal enterprises.
Nevertheless, in a statement after Mr. Trump’s tweet, Attorney General William P. Barr said the F.B.I. would ally with state and local police to identify violent protesters, whom he also called domestic terrorists.
“Groups of outside radicals and agitators are exploiting the situation to pursue their own separate, violent and extremist agenda,” Mr. Barr said. “The violence instigated and carried out by antifa and other similar groups in connection with the rioting is domestic terrorism and will be treated accordingly.”
Antifa-style protesters are hostile to law enforcement and are oriented toward street action, said Jonathan Greenblatt, the chief executive of the Anti-Defamation League, so these demonstrations provide an opportunity even if their goals are not the same as those of the original protesters. Still, he cautioned that in such a complex stew, not all the vandalism can be attributed to any one faction.
“While we have seen tactics they embrace at times during these protests, it is also unclear how many of the individuals committing violence or destroying property are antifa or just upset with the ongoing issue of police brutality,” he said.
Analysts noted a range of participants.
“We’re going to see a diversity of fringe malefactors,” said Brian Levin, the director of the Center for the Study of Hate and Extremism at California State University, San Bernardino. “We know for a fact there have been far-right agitators both online and at these rallies, as well as far-left.”
Far-right adherents generated an avalanche of posts on social media in recent days suggesting the unrest was a sign that the collapse of the American system they have long awaited was at hand. These groups, known as “accelerationists,” attempt to promote any circumstances that might speed that goal.