In Commonwealth v. Perry, the Supreme Judicial Court described the warrant requirement for seizing evidence of cellular tower dumps. As opposed phone warrants or location information warrants for a single phone user, tower dumps allow the police to obtain the user info of every person whose phone connected to a particular cell tower within a specified time period. This information, which identifies thousands of users, can then be cross-referenced with tower dumps on different dates and/or locations, to obtain information of otherwise unknown serial offenders (as was done in the Perry case). One can imagine the data being used under other circumstances to identify a particular offender and then track them over time.
The SJC held that the application for the warrant must state more than the fact that people generally carry cell phones. It must provide “a substantial basis to conclude that the defendant used his or her cellular telephone during the relevant time frame, such that there is probable cause to believe the sought after cellular information will produce evidence of the crime.”
Prior cases that have addressed the sufficiency of warrant applications have generally viewed the showing that the suspect had a cell phone and that there was probable cause that they had committed the offense under investigation as sufficient to secure a search warrant. Now, following Perry, the warrant application must show that the suspect was carrying or using the phone. Unfortunately, the SJC ruled that the decision does not apply retroactively, so prior ruling that did not require evidence of usage will not be corrected based on this ruling.