Which agency works to secure borders against terrorists and criminals?

The agency tasked with securing borders against terrorists and criminals is the one specifically focused on border security and enforcement. Border Patrol is a component of Customs and Border Protection (CBP) and operates under the Department of Homeland Security. Its primary mission is to detect and prevent illegal entries into the United States, particularly between official ports of entry. This includes the apprehension of individuals attempting to cross the border unlawfully and identifying potential threats from terrorists and criminal organizations.

While Customs and Border Protection handles a broader range of responsibilities including customs enforcement, trade regulation, and immigration enforcement at ports of entry, the Border Patrol's specific focus on patrolling and securing the borders makes it the correct choice in this context.

The FBI, while involved in national security and counterterrorism efforts, does not focus primarily on border enforcement. Immigration and Customs Enforcement (ICE) deals more with the enforcement of immigration laws within the country and the investigation of criminal activities related to immigration and customs rather than border security itself. Thus, Border Patrol is the agency that is most directly engaged in securing the borders against potential threats and criminal activity.