At more than 400 pages, the study amounted to the most searching look ever at the government’s secretive system for carrying out national-security surveillance on American soil. And what the report showed was not pretty.

The Justice Department’s independent inspector general, Michael E. Horowitz, and his team uncovered a staggeringly dysfunctional and error-ridden process in how the F.B.I. went about obtaining and renewing court permission under the Foreign Intelligence Surveillance Act, or FISA, to wiretap Carter Page, a former Trump campaign adviser.