(New York, NY) — PEN America on Tuesday signed an open letter with the National Coalition Against Censorship, Defending Rights and Dissent, and 43 other First Amendment organizations denouncing the practice of federal law enforcement agents arbitrarily stopping anti-racist protesters and whisking them away, often in unmarked vehicles, without individualized probable cause.

Federal agents from various law enforcement units, including U.S. Marshals and Customs and Border Protection, descended on Portland, Oregon earlier this month purportedly to defend federal buildings, but recent video evidence has shown the agents—often wearing no official insignia—grabbing protesters off the street presumably without probable cause. Organized by the National Coalition Against Censorship and Defending Rights & Dissent, the letter condemns the tactic as “designed to intimidate protesters into surrendering their First Amendment rights.”

“The continued deployment of federal forces in Portland, against the wishes of local elected officials, is the latest example of the Trump administration’s policy of demonizing the protesters and peddling conspiracy theories about outside agitators,” the letter reads. Additionally, such deployment of federal forces has also led to an escalation of violence. On July 6, a U.S. Marshal shot a protester in the head with an “impact munition,” fracturing his skull and leaving him critically injured.

“What these officers have done in rounding up protesters and holding them without cause is deplorable and wholly unconstitutional,” said Nora Benavidez, PEN America’s director of U.S. free expression programs. “Congress must investigate these unconstitutional practices, which are uncomfortably similar to the actions of autocratic governments around the world. Peaceful protest is a critical form of expression, and no matter the political bent of the protesters, it is the government’s responsibility to defend that right, not infringe on it. ”

These tactics—and the push to silence protesters across the country—are not new. Read PEN America’s comprehensive report on efforts to criminalize protest, Arresting Dissent.