Each piece of information in this file represents the precise location of a single smartphone over a period of several months in 2016 and 2017. The data was provided to Times Opinion by sources who asked to remain anonymous because they were not authorized to share it and could face severe penalties for doing so. The sources of the information said they had grown alarmed about how it might be abused and urgently wanted to inform the public and lawmakers.

After spending months sifting through the data, tracking the movements of people across the country and speaking with dozens of data companies, technologists, lawyers and academics who study this field, we feel the same sense of alarm. In the cities that the data file covers, it tracks people from nearly every neighborhood and block, whether they live in mobile homes in Alexandria, Va., or luxury towers in Manhattan.

One search turned up more than a dozen people visiting the Playboy Mansion, some overnight. Without much effort we spotted visitors to the estates of Johnny Depp, Tiger Woods and Arnold Schwarzenegger, connecting the devices’ owners to the residences indefinitely.