The Department of Homeland Security sent officers to Portland during protests against police brutality. Videos taken by bystanders show unidentified federal agents grabbing individuals and pulling them into unmarked vans. Homeland Security Acting Deputy Director Ken Cuccinelli confirmed the agency used unmarked vehicles to detain people, but said it did so to take them to a "safe location for questioning."
Portland officials said they did not know about the federal action. Those detained said they were taken to the city's federal courthouse before being released.
The letter, addressed to U.S. Attorney General William Barr and Department of Homeland Security Acting Secretary Chad Wolf, said the deployment was "unprecedented and violates fundamental constitutional protections and tenets of federalism."
"This abuse of power cannot continue," the letter reads.
Walsh said the deployment has inflamed the situation in Portland.
"It's being done with no communication," he said during a press conference on Tuesday. "It's being done with no regard for the rights or safety of protesters, and it appears to be needlessly escalating situation."
The letter's signatories, which include mayors from more than a dozen major cities, also sent a missive to Congress urging an investigation into the use of federal agents in American cities.
"The administration’s deployment of federal forces shows a shocking disregard for the legitimate use of our U.S. military and federal resources, as well as the authority of local law enforcement," the mayors wrote in the letter to Congress. "We are a nation of laws and fundamental constitutional guarantees."
Earlier this week, President Trump said officers may be sent to other cities, including Chicago, New York City and Philadelphia.
Walsh on Monday told WBUR's Radio Boston that he did not believe Boston was among the targeted cities, but would remain watchful.
"Certainly, the president has put a lot of threats out there across the country over the last three-and-a-half years of his presidency," Walsh said.
The U.S. Marshall's office in Boston declined to comment.
With reporting from WBUR's Deborah Becker, Roberto Scalese and Shannon Dooling