Do we know this for a fact? The only way to find this out is to test it in courts. There could be executive orders involved, the Patriot Act or related acts, or simply a "state of emergency" or two declarations:
https://en.wikipedia.org/wiki/List_of_national_emergencies_i...
Open:
"Now, Therefore, I, George W. Bush, President of the United States of America, by virtue of the authority vested in me as President by the Constitution and the laws of the United States, I hereby declare that the national emergency has existed since September 11, 2001, and, pursuant to the National Emergencies Act (50 U.S.C. 1601 et seq.), I intend to utilize the following statutes: sections 123, 123a, 527, 2201(c), 12006, and 12302 of title 10, United States Code, and sections 331, 359, and 367 of title 14, United States Code."
https://www.govinfo.gov/content/pkg/WCPD-2001-09-17/pdf/WCPD...