The United States Border Patrol is a federal law enforcement agency within U.S. Customs and Border Protection (CBP), an agency of the Department of Homeland Security (DHS). Its 11,000 agents are primarily responsible for immigration and border law enforcement as codified in the Immigration and Nationality Act. Their duty is to prevent terrorists and terrorist weapons from entering the United States and to deter, detect, and apprehend illegal immigrants and individuals involved in the illegal drug trade who enter the United States other than through designated ports of entry.