Updated at 2:02 p.m.

The U.S. Attorney for D.C. fired back at D.C. Mayor Muriel Bowser on Tuesday, rebutting her claims that the office has lacked the “willingness” to prosecute protesters who allegedly attack police.

“I can assure you that we will continue to aggressively charge any and all cases presented to this Office, but facts and evidence will dictate criminal charges, not political leveraging or personal agenda,” acting U.S. Attorney for the District of Columbia Michael Sherwin wrote in a letter to Bowser.

The U.S. Attorney’s Office for D.C. has worked with MPD to bring charges in more than 125 criminal cases since the protests began in late May, Sherwin said. Included in those charges are arson, assaults on officers and civilians, the destruction of federal property, and breaking and entering, he said.

“As I am sure you are aware, without some evidence to establish probable cause of a particular arrestee’s criminal conduct — e.g., a police officer’s observation or video footage of the alleged crime — we cannot bring federal charges,” Sherwin wrote. “Surely, by your comments you are not suggesting that this Office skirt constitutional protections and due process.”

At a press conference Monday, Bowser blamed much of the friction between demonstrators and D.C. police at this weekend’s protests in downtown D.C. on “outside agitators” who came “armed for battle” with laser pointers and baseball bats. The D.C. Metropolitan Police Department used a form of pepper spray and sting balls to disperse crowds at the protests and made 19 arrests, including 14 for felony rioting. Bowser called out the U.S. Attorney’s Office for not yet prosecuting the bulk of these individuals.

“When we make arrests for violent protests, we need those violent agitators to be prosecuted. When we arrest people with felony charges, we need the U.S. Attorney for the District, who is a federal appointee of the president, to prosecute them,” Bowser said at the press conference. “Right now there’s no accountability for people who came to these protests and attacked our police. And we haven’t seen a willingness from the U.S. Attorney to prosecute them.”

Bowser also sent a letter to Sherwin Monday asking him to “fulfill” his duty as a “public safety partner.” In the letter, she cites the 19 arrests MPD made this weekend and expresses concern that federal prosecution of the arrests is still pending.

“Given the stakes for demonstrators seeking to protest peacefully and our officers who are being assaulted while seeking to preserve the peace, I again urge you in the strongest possible terms to move forward with these cases,” Bowser wrote.

Protesters over the weekend used smoke bombs, fireworks, and “other munitions” against police, and also volleyed bricks and urine-filled balloons, D.C. Police Chief Peter Newsham said at the press conference. About a dozen MPD officers were injured.

Of the 19 arrested, the USAO has charged only one, Sherwin said, as the MPD has “failed to provide the bare minimum of articulable facts linking the charged persons with alleged individual criminal conduct.” He also likens the MPD’s lack of evidence for the weekend’s arrests to a similar dearth earlier this month, when D.C. police arrested 42 individuals Aug. 13-14. The USAO only prosecuted one of those arrested because the documents presented by D.C. police “lacked sufficient probable cause” to bring criminal charges, Sherwin said.

“Simply put, we cannot charge crimes on the basis of mere presence or guilt by association,” he wrote.

The mayor’s office did not immediately respond to a request for comment.

The District government and the U.S. Attorney’s Office of D.C. have teamed up before to prosecute protesters they deemed violent. D.C. police arrested some 230 protesters at President Donald Trump’s inauguration, and federal prosecutors charged them with a series of felonies, including rioting, despite not having specific evidence that the defendants committed the property damage. Prosecutors failed to secure any guilty verdicts in jury trials, which ended in acquittals and hung juries before the rest of the charges were dropped two years later. That defeat led Newsham to call for a potential change to the rioting statute. And it appears to be influencing the prosecutorial decisions the USAO’s office is making this summer.

Sixteen percent of the 541 people arrested and processed for activity at D.C. demonstrations this summer have been arrested by MPD for felony rioting, but the USAO has declined to pursue those rioting charges. “I think that’s a very gray area, a very dangerous area that bleeds into protesting,” Sherwin told the Washington Post in June.

Sherwin also takes issue with Bowser’s claim that the office is not “holding accountable” those who assault police officers; of the arrests MPD made August 27-28, the USAO charged four of the five criminal cases they were brought, and three individuals were charged with assaulting police, Sherwin said.

The USAO will meet with MPD leadership and U.S. Attorney General William Barr for a working session Wednesday. In his letter, Sherwin invited Bowser to attend.

This story has been updated with additional context.