The Sixth Circuit Court of Appeals has become the latest federal appeals court to consider the question of whether law enforcement needs a warrant before it obtains cell phone location data. We have (with allies) filed an amicus brief in this case, as we did in cases now pending in the Fourth and Eleventh Circuits. Clearly this is an issue that is headed toward the Supreme Court (especially if the circuit courts come to different conclusions).

In this case, United States v. Carpenter, police obtained four months of historical cell phone location records for one suspect, Timothy Carpenter, and nearly three months of records for another suspect, Timothy Sanders. They did so without getting a warrant. For Mr. Carpenter, the records contain 12,898 separate points of location data, and for Mr. Sanders, 23,034 location points—an average of one every six minutes.

In this case as in the others, the data acquired by police provides a stark demonstration of how location data can reveal extraordinarily private details about our lives. For example here we found that:

In the early afternoon on a number of Sundays, Mr. Carpenter made or received calls from the cell tower sectors nearest to his church. His cell phone records do not routinely show him in that area on other days of the week, leading to the inference that he was worshipping at those times.

Mr. Carpenter’s call records reveal that over the course of four months, his phone was located in more than 200 separate cell tower sectors. On one day, he made and received 141 calls while located in 40 different sectors. These records provide a granular accounting of Mr. Carpenter’s locations and movements over time.

From December 23 to December 27, 2010, Mr. Carpenter’s last call of the night and first call of the next morning were either or both placed from the sector nearest to his home, suggesting that he slept at home on those evenings. But on the night of December 22, 2010, his last call of the night and first call of the next morning were placed from a Detroit neighborhood several miles away from his home. Although we have no reason to believe it to be the case here, Carpenter’s data shows how such information could easily reveal private details about who a person is sleeping with.

Records revealing these kinds of sensitive details of our lives are exactly what the Fourth Amendment was intended to protect. It’s crucial that law enforcement go to a neutral judge, demonstrate probable cause, and get a warrant before delving into a transcript of our every movement over the course of weeks or months.

Our brief was joined by the ACLU of Michigan, Brennan Center for Justice, Center for Democracy & Technology, Electronic Frontier Foundation, and National Association of Criminal Defense Lawyers.