"The U.S. Customs and Border Protection (CBP) Carrier Information Guide is designed to serve as a reference aid for travel industry personnel. It outlines the various documentary requirements that apply to persons entering or departing the United States. Carrier personnel are urged to familiarize themselves with all sections of the publication and reference it in their examination of travel documents." "Anyone seeking entry into the United States of America, whether a United States ('U.S.') citizen, a U.S. resident or a visitor to the U.S., must have in his/her possession documentation of identity and nationality. In addition, each traveler must be in possession of proper documentation for the purpose of his/her travel. U.S. law provides that transportation carriers may be liable for improperly documented passengers brought to the United States."
U.S. Customs and Border Protection: http://www.cbp.gov/