New York area airports (JFK, LaGuardia, and Newark) were closed again and reopened the next morning. The only traffic from LaGuardia during the closure was a single C-9C government VIP jet, departing at approximately 5:15 p.m. on the 12th. Civilian air traffic was allowed to resume on September 13, 2001, with stricter airport security checks, disallowing for example the box cutting knives that were used by the hijackers. (Reinforcement of cockpit doors began in October 2001, and was required for larger airlines by 2003.) First, stranded planes were allowed to fly to their intended destinations, then limited service resumed. The backlog of delayed passengers took several days to clear. Due to a translation error, controllers believed Korean Air Flight 85 might have been hijacked. Canadian Prime Minister Jean Chrétien and U.S. authorities ordered the United States Air Force to surround the plane and force it to land in Whitehorse, Yukon and to shoot down the plane if the pilots did not cooperate. Alaska Governor Tony Knowles ordered the evacuation of large hotels and government buildings in Anchorage. Also in Alaska at nearby Valdez, the U.S. Coast Guard ordered all tankers filling up with oil to head out to sea. Canadian officials evacuated all schools and large buildings in Whitehorse before the plane landed safely. Precautionary building closings and evacuations Many businesses across the United States closed after the intentional nature of the events became clear, and many national landmarks and financial district skyscrapers were evacuated out of fear of further attacks. United States United Nations Headquarters Building in New York City Most skyscrapers in New York City (including the Chrysler Building and Empire State Building, which was evacuated several times on September 11 and after due to false reports of potential threats), Chicago (including Sears Tower) and Philadelphia The Washington Monument in Washington, D.C. The Statue of Liberty, Ellis Island and ferries to both in and out of New York City All state capitols and many government buildings surrounding the capitols Many landmarks in the United States, including the Mall of America outside of Minneapolis, Minnesota and the Space Needle and Columbia Center in Seattle, Washington All federal buildings in Washington, D.C., including the White House, the United States Capitol, the United States Supreme Court Building and Blair House. Across the country, approximately one million federal workers were sent home The D.C. Department of Motor Vehicles (although all other D.C.