The USA Patriot Act, passed in October 2001, expanded the federal system of obtaining authority to use surveillance tools in foreign intelligence cases to some criminal cases. Here's a comparison of the changes:
Criminal cases pre-October 2001
Source of authority for search warrant, wiretap or other surveillance tool: A U.S. District Court judge, such as Portland Seven Judge Robert Jones.
Requirement: Written representation that investigators have probable cause to believe that a crime has been committed and that evidence of that crime will be found in the place to be searched.
Accountability: Investigators are required to report back to the court on what they discover and, in the case of search warrants, to notify the person whose property was searched that the search has taken place.
Criminal cases post-October 2001
Source of authority for search warrant, wiretap or other surveillance tool: A special court, called a FISA court, which consists of federal court judges.
Requirement: Proof of probable cause is generally not required. But investigators must have certification, from the U.S. attorney general, that 'a significant purpose' of their investigation is to gather intelligence.
Accountability: The FISA court does not evaluate the government's certification or representations. Investigators are not required to report back to the court on what they discover or to let people know that their premises have been entered for the purpose of planting eavesdropping devices or for other investigative reasons.