A rare trial began Monday in a class action challenging the placement of unauthorized migrant children in adult detention facilities once they turned 18.

The bench trial, expected to last three weeks, comes in a case brought by Kirkland & Ellis and the National Immigrant Justice Center against the U.S. Department of Homeland Security and its unit, U.S. Immigration and Customs Enforcement. Plaintiffs allege that government officials routinely violate the federal Trafficking Victims Protection Reauthorization Act, which, under a 2013 amendment, requires that authorities place unaccompanied migrant children who turn 18 in the “least restrictive setting available.”