Existing Fourth Amendment law speaks to such searches only in limited contexts, however. The Fourth Amendment requires the government to obtain a search warrant when looking through a person’s hard drive or private e-mail, for example. It also requires that the warrant specify the type of evidence the government is seeking. It may also require a warrant or a subpoena to collect information that is inside a database. However, if the government collects data in compliance with the Fourth Amendment, and then it aggregates the data into a database, the process of searching through the database is not itself regulated by the Fourth Amendment. Even if the government violates the Fourth Amendment when collecting the data, the data may be stored, aggregated, and used for any purpose other than that for which the data were wrongfully accessed. So, for example, the Court has allowed records illegally seized by criminal investigators to be used by tax investigators on the basis that restricting the subsequent use would not deter the original unconstitutional conduct.7
Broad machine-aided searches and the government’s reuse of lawfully or unlawfully obtained data raise very important questions of public policy. What standards should govern access to or use of data that has already been collected? Should use of databases or specific analytical techniques such as data mining be regulated at all? If querying a database or running a data mining program on a database constitutes a search, when is such a search “reasonable”? Must the police have a specific individual in mind before searching a database for information on him or her? In the absence of clear standards or guidelines to govern their conduct or even to help them make reasonable judgments, the police cannot do their work. Moreover, what level of legal authorization should guide database queries? If a legal standard is used, is relevance the right standard? Or is something more like reasonable suspicion or probable cause the proper standard to use?
The Fourth Amendment applies to searches and surveillance conducted for domestic law enforcement purposes within the United States, and those conducted outside of the United States if they involve U.S. citizens (although not necessarily permanent resident aliens). In a 1972 case commonly referred to as the Keith decision, the Supreme Court held that the Fourth Amendment also applies to searches and surveillance conducted for national security and intelligence purposes within the United