So-called sanctuary city policies are a “public nightmare” that make American cities more dangerous, Rep. James Comer said Wednesday, opening a congressional hearing with mayors from four major cities the Trump administration says are hindering its mass deportation agenda and protecting people who are in the United States illegally.

The mayors of four so-called “sanctuary cities” faced tough questions from lawmakers about their policies toward immigrants and how they work with Immigration and Customs Enforcement at a House Oversight and Government Reform Committee on Wednesday.