An estimated 11 million unauthorised aliens reside in the United States, and this population is estimated to increase by 500,000 annually. Each year, approximately 1 million aliens are apprehended trying to enter the United States illegally. Although most of these aliens enter the United States for economic opportunities and family reunification, or to avoid civil strife and political unrest, some are criminals, and some may be terrorists. All are violating the United States' immigration laws.