Border enforcement is a core element of the Department of Homeland Security's (DHS's) effort to control illegal migration, with the U.S. Border Patrol (USBP) within the Bureau of Customs and Border Protection (CBP) as the lead agency along most of the border. Border enforcement has been an ongoing subject of congressional interest since the 1970s, when illegal immigration to the United States first registered as a serious national problem; and border security has received additional attention in the decade following the terrorist attacks of 2001.