Legality questioned of mass-surveillance tool that warrantlessly tracks US citizens and their 'patterns of life'

The cellphone tracking tool Fog Reveal has been utilized by law enforcement agencies across the U.S. to track American citizens and study their movement. The tool — to which access is secured by way of subscription — helps investigators and others bypass cellphone companies and internet service providers when seeking users' historical GPS data. Fog Reveal has also been used to track citizens without search warrants, prompting some to question whether it violates the Fourth Amendment.

Fog Data Science is a Virginia-based company founded by Robert Liscouski, the former head...