The judges who rule on Foreign Intelligence Surveillance Act (FISA) requests oversee the nation’s most sensitive national security cases, and their warrants are some of the most closely guarded secrets in the world of U.S. law enforcement and intelligence gathering. Any FISA application has to be approved at the highest levels of the Justice Department and the FBI.

Applications for FISA warrants, Comey said, are often thicker than his wrists, and that thickness represents all the work Justice Department attorneys and FBI agents have to do to convince a judge that such surveillance is appropriate in an investigation.

The government’s application for the surveillance order targeting Page included a lengthy declaration that laid out investigators’ basis for believing that Page was an agent of the Russian government and knowingly engaged in clandestine intelligence activities on behalf of Moscow, officials said.