NORTHWESTERN UNIVERSITY LAW REVIEW Notably, all four categories mirror the distinction between envelope and content information. The envelope provides addressing information, and the content provides the actual communication that the network will deliver to its destination B. Prospective Versus Retrospective Surveillance The next distinction considers the timing of the surveillance. Is the surveillance designed to capture future communications that have not yet been sent over the network("prospective " surveillance), or is it designed to look for stored records and past communications that may be retained in the network("retrospective"surveillance)? Wiretapping a telephone provides the classic example of prospective surveillance. When the FBI wiretaps a telephone line, it seeks to listen to the contents of future conversations. In the case of retrospective surveillance, in contrast, the government seeks to access stored records of past communications. The use of o.J. Simpsons telephone records in his murder trial furnishes a well-known example. 35 The Los Angeles Police Department obtained Simpsons phone records to show that Simpson had made several suspicious calls the night of his wifes murder. b This example illustrates retrospective surveillance of envelope information; the police used the phone company's stored business records lating to past communications to try to prove Simpson's guilt The law often distinguishes between prospective and retrospective sur veillance because they raise somewhat different privacy concerns. As Jus- tice Douglas noted in his concurrence in Berger v. New York, 8 prospective surveillance can at worst constitute a dragnet, sweeping in all conversation within its scope. 39 The surveilling party taps into the network at a given location and picks up traffic passing through, but cannot know in advance exactly what the traffic will be. Some of the traffic may prove relevant, but usually much of the traffic will not be. 40 Further, it can be technically diffi- cult(if not impossible) to filter the communications down to the relevant evidence before the government observes it. Accordingly, prospective sur- veillance tends to raise difficult questions of how the communications should be filtered down to the evidence the government seeks. 4 In con- trast, the scope of retrospective surveillance is generally more limited. The 35 See Michael Miller, Time of Phone Call a Key to O.J. Case, S F. EXAMINER, Aug. 13, 1994, at 36 38 388 U.S. 41, 64-68(1967)(Douglas, J, concurring) See scott v ur 436 U.S. 128, 145(1978)(Brennan, J, dissenting)("Because it is dif- ficult to know with of certainty whether a given communication is subject to interception prior to its interception sarily must be a margin of error permitted. See id. at 140-43(discussing difficulties of filtering the fruits of a wiretap) 616N O R T H W E S T E R N U N I V E R S I T Y L A W R E V I E W 616 Notably, all four categories mirror the distinction between envelope and content information. The envelope provides addressing information, and the content provides the actual communication that the network will deliver to its destination. B. Prospective Versus Retrospective Surveillance The next distinction considers the timing of the surveillance. Is the surveillance designed to capture future communications that have not yet been sent over the network (“prospective” surveillance), or is it designed to look for stored records and past communications that may be retained in the network (“retrospective” surveillance)? Wiretapping a telephone provides the classic example of prospective surveillance. When the FBI wiretaps a telephone line, it seeks to listen to the contents of future conversations. In the case of retrospective surveillance, in contrast, the government seeks to access stored records of past communications. The use of O.J. Simpson’s telephone records in his murder trial furnishes a well-known example.35 The Los Angeles Police Department obtained Simpson’s phone records to show that Simpson had made several suspicious calls the night of his wife’s murder.36 This example illustrates retrospective surveillance of envelope information; the police used the phone company’s stored business records relating to past communications to try to prove Simpson’s guilt.37 The law often distinguishes between prospective and retrospective sur￾veillance because they raise somewhat different privacy concerns. As Jus￾tice Douglas noted in his concurrence in Berger v. New York, 38 prospective surveillance can at worst constitute “a dragnet, sweeping in all conversation within its scope.”39 The surveilling party taps into the network at a given location and picks up traffic passing through, but cannot know in advance exactly what the traffic will be. Some of the traffic may prove relevant, but usually much of the traffic will not be.40 Further, it can be technically diffi￾cult (if not impossible) to filter the communications down to the relevant evidence before the government observes it. Accordingly, prospective sur￾veillance tends to raise difficult questions of how the communications should be filtered down to the evidence the government seeks.41 In con￾trast, the scope of retrospective surveillance is generally more limited. The 35 See Michael Miller, Time of Phone Call a Key to O.J. Case, S.F. EXAMINER, Aug. 13, 1994, at A1. 36 See id. 37 See id. 38 388 U.S. 41, 64–68 (1967) (Douglas, J., concurring). 39 Id. at 65. 40 See Scott v. United States, 436 U.S. 128, 145 (1978) (Brennan, J., dissenting) (“Because it is dif￾ficult to know with any degree of certainty whether a given communication is subject to interception prior to its interception, there necessarily must be a margin of error permitted.”). 41 See id. at 140–43 (discussing difficulties of filtering the fruits of a wiretap)