"Since the September 11, 2001 terrorist attacks, considerable concern has been raised because the 19 terrorists were aliens who apparently entered the United States with temporary visas despite provisions in immigration laws that bar the admission of terrorists. Foreign nationals not already legally residing in the United States who wish to come to the United States generally must obtain a visa to be admitted, with certain exceptions noted in law. Fears that lax enforcement of immigration laws regulating the admission of foreign nationals into the United States may continue to make the United States vulnerable to further terrorist attacks have led to revisions in the policy as well as changes in who administers immigration law. The 107th Congress expanded the definition of terrorism and the designation of terrorist organizations used to determine the inadmissibility and removal of aliens in the USA Patriot Act (P.L. 107-56). Another law, the Enhanced Border Security and Visa Entry Reform Act (P.L. 107-173), sought to improve the visa issuance process by mandating data sharing so that consular officers have access to relevant electronic information. This law also required the development of an interoperable electronic data system to be used to share information relevant to alien admissibility and removability and required that all visas issued by October 2004 have biometric identifiers."
CRS Report for Congress, RL31512
U.S. Department of State, Foreign Press Center: http://www.fpc.state.gov/