eral law defines “probable cause” to mean “a belief that an individual is committing, has committed, or is about to commit a particular offense” and that the information sought is germane to that crime.¹ The Supreme Court generally requires that the government provide the subject of a search with contemporaneous notice of the search.²

Collecting information from a person constitutes a search if it violates that individual’s reasonable expectation of privacy. The Supreme Court has held that a person has a reasonable expectation of privacy in their homes, sealed letters, and the contents of their telephone calls. On the other hand, the Court has determined, for example, that warrants are not required to search or seize items in the “plain view” of a law enforcement officer,³ for searches that are conducted incidental to valid arrests,⁴ or to obtain records held by a third party, even if those records are held under a promise of confidentiality.⁵ The Court has interpreted this last exception broadly to find that the Fourth Amendment is inapplicable to telecommunications “attributes” (e.g., the number dialed, the time the call was placed, the duration of the call, etc.), because that information is necessarily conveyed to, or observable by, third parties involved in connecting the call.⁶

Moreover, the Fourth Amendment poses no limits on how the government may use information, provided that it has been obtained legally, and some limits on the use of data obtained illegally. Consequently, personal data seized by the government in compliance with the Fourth Amendment may later be used in a context for which the data could not have been obtained lawfully. The rest of this section addresses two important examples of areas in which the evolution of technology and new circumstances suggest that current Fourth Amendment law and practice may be outdated or inadequate.

In some ways, machine-aided searching of enormous volumes of digital transaction records is analogous to a general search, especially if those records contain highly sensitive information. Much like a general search in colonial times was not based on specific evidence or limited to a particular person or place, a machine-aided search through digital databases can be very broad.