According to a September 13, 2001, news release from the U.S. Department of Transportation: “Secretary of Transportation Norman Y. Mineta has ordered the national airspace system re-opened to commercial aviation, effective at 11 a.m. Eastern time Thursday [September 13, 2001].”
– Dept. of Transportation Press Release, “September 11, 2001: Attack on America: Statement of U.S. Secretary of Transportation Norman Y. Mineta; September 13, 2001,” U.S. Department of Transportation Federal Aviation Administration, Sep. 13, 2001