Liquefied natural gas
Immediately after the September 11, 2001 attacks, the Coast Guard imposed restrictions on traffic in American waters. Vessels over 300 tons displacement must file notice within 96 hours of estimated time of arrival in American waters, or 24 hours for short voyages. Liquefied natural gas carriers are forbidden to enter American waters without escort and to anchor near major cities. Coast Guard and Auxiliary units patrol key harbors and waterfronts and intercept foreign merchant vessels for identification and crew checks. The Coast Guard stepped up patrols in waters near New York City and Washington in 2004 after receiving reports of increased threats.