A search warrant is issued by a judge if she finds, based on a sworn affidavit submitted by a law enforcement officer, that there is probable cause to believe
that a crime has been or is being committed and that the search will uncover evidence concerning the crime.
A subpoena is issued without judicial approval, by a prosecutor or law enforcement official (such as an FBI agent) who claims that the information sought is relevant to an ongoing investigation.
A search warrant permits immediate seizure -- the government comes in and goes through your files or unplugs your hard drive and takes it away -- while a subpoena directs the recipient to bring forth the material at some future date (e.g. within 10 days), giving the recipient time to go through its records and select out what is responsive and make copies.
Subpoenas can be challenged in court, but the service provider often has no incentive to defend the privacy of its customers, especially if they need never be notified that their data has been turned over to the government. In many cases, the recipient of subpoena can, but is not required, to disclose the subpoena to its customers.
Probable cause is the highest standard for seizing evidence (it is also the standard for arresting someone or for bringing an indictment), while relevance is the lowest.
The Federal Wiretap Act (Title III) – adopted in 1968, requires a judicial order based on a finding of probable cause for most government interception, in criminal investigations, of the content of communications. Requires high level approval of applications and other protections going beyond ordinary search warrants.
Electronic Communications Privacy Act of 1986 (ECPA) – extended the Wiretap Act to cover wireless and data communications (email) in transit; established different rules for access to communications in storage; required judicial order for real-time collection of transaction data (using a pen register or trap and trace device), but under a rubber- stamp standard.
Foreign Intelligence Surveillance Act of 1978 (FISA) – for intelligence investigations, requires a judicial order based on a finding of probable cause to believe that the target of the surveillance is an agent of a foreign power (a foreign government, foreign political party, or international terrorist group); as amended, also covers physical searches, use of pen registers and trap and trace devices in intelligence matters, and access to business records.
Communications Assistance for Law Enforcement Act of 1994 (CALEA) – requires telephone companies to design their systems to be wiretap friendly; generally did not address surveillance standards (said that location information could not be obtained with a mere pen register).
The traditional search and seizure plays out on TV every night: Loud knocking on the
door. Police. Open up. We have a warrant.
The Supreme Court has held that knock
and announce
is an element of the Fourth Amendment's reasonableness standard. It can
be dispensed with, but only in special circumstances. However, the storage revolution is
taking information outside of the home or office, where the government can get it only
with notice to the person who created the data, and placing it on the servers of third
parties, from whom the government can obtain it without contemporaneous notice to the
person whose privacy is being invaded. The Wiretap Act includes some additional
procedures that compensate for the lack of contemporaneous notice when
communications are intercepted in transit, but none of those protections apply to
government seizure of stored email and documents.