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Smartphone tracking data is never completely secure. This is the conclusion that can be drawn from the recent investigation in the US. Members of Congress have opened an investigation into a data analytics company that sells people’s smartphone location data to government agencies including the FBI and Department of Homeland Security.

The company, Venntel, aggregates location data from smartphone apps including games and weather forecast apps, and in turn sells that data to its clients.

Lawmakers are pressing Venntel to release more information about its clients and data sources, according to

When people’s smartphone location data is tracked, that data is tied to an anonymous identifier rather than to personally identifying information. But multiple studies have shown that it’s easy to “de-anonymize” this data by connecting it with other data points — for example, seeing where a device stays overnight and connecting that with a person’s home address — raising the possibility that location data could be used to keep tabs on individuals’ movements.

The Trump administration has purchased access to millions of smartphone users’ location data and is using it for immigration enforcement, according to a February report from the Wall Street Journal. The report said the DHS used location data from Venntel to track down people who crossed the border into the US illegally.