If the FBI was actually, thoroughly, investigating Carter Page, they would have come across the evidence that Page was working for the US as a source independently of Clinesmith.
Likely the domestic spying gambit followed what Brennan and Clapper were already doing using CIA and National Intelligence as well as similar agencies worldwide. The FBI’s track record of 99% wins dealing with the FISA judges gave Obama (who's approval was required by law for spying on U.S. citizens) carte blanche. Both Susan Rice and Samantha Powers are in the mix as well. Their unmasking endeavors were probably part of this massive treason.