"[W]e hold that, absent express direction or authorization by federal statute or federal officials, state and local law enforcement officers may not detain or arrest an individual solely based on known or suspected civil violations of federal immigration law.
Like the district court, we conclude that the deputies seized Santos for purposes of the Fourth Amendment when Deputy Openshaw gestured for her to stay seated after dispatch informed him of the outstanding civil ICE deportation warrant. See supraPart III.C. At that time, the deputies’ only basis for detaining Santos was the civil ICE warrant. Yet as the defendants concede, the deputies were not authorized to engage in immigration law enforcement under the Sheriff’s Office’s Section 1357(g)(1) agreement with the Attorney General. They thus lacked authority to enforce civil immigration law and violated Santos’s rights under the Fourth Amendment when they seized her solely on the basis of the outstanding civil ICE warrant." - Santos v. Frederick County, Aug. 7, 2013.