"Nor would it be logical to expect any court to impose such a standard of "known", since when the government initiates an investigation, by definition, the word "investigation" admits to much being unknown about a case."
FISA is quite clear in the requirements for warrantless eavesdropping .
"Notwithstanding any other law, the President, through the Attorney General, may authorize electronic surveillance without a court order under this subchapter to acquire foreign intelligence information for periods of up to one year if the Attorney General certifies in writing under oath that—"
- the electronic surveillance is solely directed at—
- the acquisition of the contents of communications transmitted by means of communications used exclusively between or among foreign powers, as defined in section 1801 (a)(1), (2), or (3) of this title; or
- the acquisition of technical intelligence, other than the spoken communications of individuals, from property or premises under the open and exclusive control of a foreign power, as defined in section 1801 (a)(1), (2), or (3) of this title;
- there is no substantial likelihood that the surveillance will acquire the contents of any communication to which a United States person is a party; and
- the proposed minimization procedures with respect to such surveillance meet the definition of minimization procedures under section 1801 (h) of this title; and
Note that it is A, B and C, not either or. If they can't meet this requirement then they need a warrant (or if you prefer, a court order)
The whole point is that they are allowed to do so without a warrant if the targets are
known agents of a foreign power, if there is any question, then they need a warrant. The A.G. has to certify
under oath that they are in fact agents of a foreign power. There is no room for "might be", "we think they are".
First, 50 USC 1802 is entirely inapplicable to the present discussion because that section deals solely with surveillance of non-U.S. persons. And in this discussion we are specifically addressing those situations that include a U.S. persons.
Second, even if §1802 did permit surveillance of U.S. persons, it does not contain, as you assert, the word "known", nor does the language you highlighted even imply such a standard.
Third, your standard that the subject be a "known" agent of a foreign power for the surveillance to be "reasonable", is so out of touch with legal reality, that it is even a greater standard than the courts already require for regular 4th Amendment searches, where the standard of reasonableness is "probable cause".
Fourth, since the In re: Sealed Case court held that "the President did have inherent [constitutional] authority to conduct warrantless searches to obtain foreign intelligence information", then the standard of reasonableness required in such cases is clearly less than the "probable cause" needed had a warrant been required. That would mean that, at most, a standard of "reasonable suspicion" might be required.
Fifth, the issue you raised in #37 regarding pre-FISA versus post-FISA court decisions is irrelevant, since the courts (both pre-FISA and post-FISA) based their holdings on a constitutional right of the President, and Congress is forbidden from infringing on a constitutional right.