WASHINGTON (AP) — A federal judge Wednesday asked attorneys to identify witnesses and offer plans for how to conduct a contempt probe of the Trump administration for failing to turn around planes carrying Venezuelan migrants to El Salvador in March.

U.S. District Judge James Boasberg in Washington said a ruling Friday by the U.S. Court of Appeals for the District of Columbia Circuit gave him the authority to proceed with the inquiry, which will determine whether there is sufficient evidence to refer the matter for prosecution.

On March 15, Boasberg ordered the aircraft carrying accused gang members to return to the U.S., but they landed instead in El Salvador, where the migrants were held at a notorious prison.