She called for full identification of all law enforcement and military agencies involved in policing the protests in Washington, D.C.
"It is alarming that here in our nation's capital, the thousands who have turned out peacefully have been confronted with the deployment of various security officers from multiple jurisdictions, including unidentified federal law enforcement personnel," Pelosi wrote.
She said she's seen officers from the Washington, D.C., police, the federal Bureau of Prisons, the National Park Service, the National Guard and the FBI.
"We are concerned about the increased militarization and lack of clarity that may increase chaos," Pelosi said.
She called for a full list of all agencies involved in policing the protests and their individual responsibilities.
"Congress and the American people need to know who is in charge, what is the chain of command, what is the mission and by what authority is the National Guard from other states operating in the capital."
Pelosi addressed the letter during her weekly news conference in the Capitol, hours after Attorney General and FBI Director Christopher Wray held their own briefing.
Barr on Thursday said Trump asked him to lead the federal response to protests in the nation's capital, which included the clearing of Lafayette Park for the president's visit to St. John's Church on Monday.
"This is the federal city. It's the seat of the federal government. It is the responsibility of the federal government to render that protection," Barr said.
The law enforcement officials addressed the federal response to protests and violence linked to the police-involved killing of George Floyd last week in Minneapolis. The arrest that led to his death was captured on cellphone video, prompting outrage and protests across the globe.
The former officer who kneeled on Floyd's neck during his arrest, Derek Chauvin, faces a second-degree murder charge. Three other officers present during the incident were charged with aiding and abetting murder in the second-degree.
"It is impossible for any normal human being not to be struck to the heart with horror," Barr said.
The Trump administration has faced criticism after U.S. Park Police used smoke canisters and pepper balls to clear protesters from Lafayette Park in front of the White House. After the protesters were dispersed, Trump walked from the White House to St. John's Church for what some critics said amounted to a "photo op." The church experienced a fire the night before during protests.
Barr said Thursday that he meant to order the park's clearance earlier in the day to expand the security perimeter around the White House, but there weren't enough personnel in place to carry that out. The attorney general said the decision to expand the perimeter came before Trump decided to visit St. John's Church.
He said demonstrators in front of the White House were "becoming increasingly unruly" and that they had been asked to move one block away three times.
Black Lives Matter D.C. and The American Civil Liberties Union of the District of Columbia, along with other rights groups filed a lawsuit Thursday against Trump, Barr and Defense Secretary Mark Esper for the actions taken to clear the park and plaza near the White House.
"This case is about the president and attorney general of the United States order the use of violence against peaceful demonstrators who were speaking out against discriminatory police brutality targeted at black people," the lawsuit read.
Also named as plaintiffs in the lawsuit are the Lawyers' Committee for Civil Rights Under Law, the Washington Lawyers' Committee for Civil Rights and Urban Affairs, and the law firm of Arnold & Porter.
"What happened to our members Monday evening, here in the nation's capital, was an affront to all our rights," said April Goggans, core organizer of Black Lives Matter D.C. "The death of George Floyd and Breonna Taylor at the hands of police officers has reignited the rage, pain, and deep sadness our community has suffered for generations. We won't be silenced by tear gas and rubber bullets. Now is our time to be heard."
Barr blamed recent violent activity during protests on "antifa" -- anti-fascist groups deemed a terrorist organization by the administration this week -- and a "witches' brew" of other groups.
"We have evidence that antifa and other similar extremist groups, as well as actors of a variety of different political persuasions have been involved in instigating and participating in the violent activity," he said. "And we are also seeing foreign actors playing all sides to exacerbate the violence."
Wray said he was "appalled and profoundly troubled" by the video of Floyd's death, saying his death "erodes the trust" people have in law enforcement.
"This is not just about George Floyd," he said. "This is about all of those over the years who have been unjustifiably killed or had their rights violated.