WASHINGTON (AP) — The two black SUVs travel from checkpoint to checkpoint, each guarded by federal drug enforcement agents alongside members of the National Guard.

Just down the street from the White House, acting Drug Enforcement Administrator Timothy Shea hops out to greet his agents who have been working 12- to 14-hour shifts to prevent any more of the violence that has erupted in Washington during protests over the police death of George Floyd in Minneapolis.

The more than 100 DEA agents who have been deployed in the district are part of a massive federal response by the Trump administration aimed at quelling the violence. Scores of heavily armed federal officers in tactical gear have been on the streets for days, after demonstrators set fires, broke store windows and stole items from the shelves and left police officers injured.

"We have to respect protesters, but we can't tolerate criminal activity associated with that," Shea said Wednesday, after reminding a group of agents that the demonstrations could go late into the night.

For Shea and others who have been working around-the-clock, splitting their time between the street and a Justice Department command center, their agents are serving a vital purpose by helping to relieve the Metropolitan Police Department so it can handle other emergencies that could arise.

Over the course of a few hours, Shea visited a handful of checkpoints throughout downtown Washington that have been set up to help slow the numbers of vehicles coming into the area. Investigators believe vans are being brought in, loaded with bricks and rocks to be thrown at police. On Wednesday, investigators uncovered a full pallet of bricks they believe demonstrators were planning to use to throw at officers, a law enforcement official said.

"They've been verbally assaulted at times. Earlier in the week there were bricks thrown at their vehicles, and they responded with restraint and they respected the rights of protesters but were also able to identify criminal activity that was happening," Shea said of his agents.

Many of the dozens of agents who met with Shea said some demonstrators had shouted obscenities, but Wednesday night's protests remained largely peaceful. Earlier in the day, people walking by had thanked them for being there, multiple agents said.

The Trump administration has made an effort to show a collection of force in Washington, manning the checkpoints and lining some streets with armored military vehicles.

The Justice Department had deployed agents from every one of its agencies, including the FBI's Hostage Rescue Team, an elite tactical unit, and riot teams from the Bureau of Prisons.

The Park Police and Secret Service have had dozens of officers out in riot gear for the last few nights, in addition to the Metropolitan Police Department. U.S. Customs and Border Protection and other Homeland Security agencies were also dispatched.

Most of the protesters have been peaceful and tried to discourage violence. President Donald Trump, Attorney General William Barr and others have tried to blame some of the civil unrest on left-wing extremist groups, including antifa, and other "anarchists." Short for anti-fascists, antifa is an umbrella term for far-left-leaning militant groups that resist neo-Nazis and white supremacists at demonstrations.

A senior Justice Department official said intelligence gathered by the FBI shows that antifa and other groups have been involved in some of the protests around the U.S. There have been "multiple instances" where people who have been arrested have identified themselves to law enforcement as members of antifa, the official said. The official, who could not discuss ongoing investigations publicly and spoke on condition of anonymity, did not provide specific details about those incidents.

Investigators also believe that groups have been training some demonstrators on techniques to rush officers on the lines of the protests and giving tips on how to avoid being arrested, the official said.