Two legal groups are suing U.S. Immigration and Customs Enforcement, saying it is retaliating against immigrant hunger strikers at a Washington detention center by placing them in solitary confinement -- something the agency denies.

The American Civil Liberties Union of Washington and Columbia Legal Services said Thursday they're asking a federal judge in Tacoma for a temporary restraining order to halt the practice.

The groups say hunger-striking is free speech protected by the First Amendment.

Hundreds of detainees participated in a hunger strike about a month ago to protest the conditions at the Northwest Detention Center in Tacoma. Some began a renewed effort March 24, and activists say about 20 people have been placed in solitary confinement.

The agency says it separated some detainees from the general population because they were pressuring others into supporting the strike.