WASHINGTON — The Justice Department is urging U.S. attorneys nationwide to use federal courts — where more serious charges are available — to prosecute people suspected of violence or property damage during the protests over George Floyd’s death.

Attorney General William Barr said Thursday that federal agents have arrested 51 people for acts of violence since the protests began. The U.S. Marshals service said 21 federal courthouses have been damaged in 15 states, and the ATF said it was investigating 847 separate cases of arson and 76 incidents involving explosives.

Investigators said social media photos and videos have led to some of the arrests.

“We’re seeing people who are exploiting this situation to pursue violent, extremist agendas, anarchists like antifa and other agitators,” said FBI Director Christopher Wray.

Barr said a “witch’s brew” of extremists with many different motivations were involved. On Wednesday, authorities revealed that the FBI arrested three members of a radical right-wing group called Boogaloo in Las Vegas, accusing them of planning to attack police during the demonstrations.

Law enforcement officials say in many cases, people travel from other states to reach the scene of protests, then rely on informal networks to supply them with bricks, gasoline and advice on where to strike.

“We have absolutely seen outside agitators across the country,” said Zach Terwilliger, the U.S. attorney from northern Virginia who is helping to coordinate the federal response.

He said he was aware of at least two cases of violence caused by members of antifa, which is a loosely organized radical leftist group, though he declined to say where they occurred.

“The violence and property damage is being caused by a combination of opportunistic looters and plain thugs taking advantage of the situation,” Terwilliger said.

Investigators said social media photos and videos as well as tips from the public have led to some of the arrests.

In Nashville, 25-year-old Wesley Somers was charged with helping to set fire to the city hall building. Court documents said he was identified through video clips and pictures posted by people nearby.

Prosecutors said photos taken by a witness led to the arrest of 31-year-old Timothy O’Donnell, charged with setting fire to a police vehicle in Chicago.

A 28-year-old Illinois man, Matthew Rupert, was arrested after investigators said they found a two-hour video posted to his Facebook account.

“In the video, Rupert passes out explosives he indicates he possessed, encourages others to throw his explosives at law enforcement officers, actively damages property, appears to light a building on fire, and loots businesses in Minneapolis,” court documents said.

Prosecutors in Buffalo, New York, said video helped identify 20-year-old Courtland Renford, charged with throwing a burning laundry basket into city hall.

“Many have suffered from the violence instigated through these radicals and extremists, including members of our own law enforcement family — officers killed or gravely injured while just doing their jobs, fulfilling their duty to the public by trying to keep everyone safe,” Wray said.