WASHINGTON — Paramilitary enforcers for Mexican drug cartels are responsible for a wave of violence in Nuevo Laredo that poses a serious threat for residents on both sides of the Southwest border, U.S. law enforcement officials told a House committee Thursday. Assassinations, kidnappings and daylight shootouts between military-trained gangs place citizens at risk along the border where violence has soared past historical norms, officials said. "These paramilitary groups work for the cartels as enforcers and are a serious threat to public safety on both sides of the border," said Chris Swecker, the FBI assistant director for the criminal investigative division....