“The Fourth Amendment protects us from “unreasonable” government searches of our persons, houses, papers and effects. How courts should determine what is and isn’t reasonable in our increasingly digital world is the subject of a new amicus brief we filed today in San Francisco federal court.  At issue is historical cell site data—the records of the cell towers a customer’s cell phone connects to. The government has long maintained that it’s unreasonable for customers to expect those records to remain private. As a result, the government argues it does not need a search warrant to obtain historical cell site records from cell phone providers.  Federal appeals courts are divided on the issue. In 2013, the Fifth Circuit Court of Appeals, which covers Louisiana, Mississippi and Texas, ruled there was no expectation of privacy in historical cell site data. But last month, the Eleventh Circuit Court of Appeals, which covers Alabama, Florida and Georgia, reached the opposite conclusion, ruling people did have an expectation of privacy in this information.  Federal magistrate judge Nathanael Cousins in San Francisco, who is not required to follow either the Fifth or Eleventh Circuit–he’s bound to follow the Ninth Circuit which hasn’t ruled on the issue yet–recently requested the local U.S. Attorney’s office to explain why the government believed it did not need a search warrant to obtain cell site records. He invited the San Francisco Federal Defender to file a response as well, and we filed an amicus brief supporting a warrant requirement. The ACLU of Northern California and University of San Francisco law professor Susan Freiwald and EFF special counsel Marcia Hofmann also submitted amicus briefs. A Fourth Amendment “search” is an intrusion upon something in which a person has a subjective expectation of privacy that society considers reasonable. By definition, determining whether a search is “reasonable” requires looking at what society considers to be deserving of privacy protection. So our amicus brief explains why many Americans actually expect this detailed and sensitive location information to remain private, even when it’s stored by phone companies.”