Major U.S. cities are spending millions of dollars on tools that track and extract data from people's cellphones — but almost nothing on rules to guide their use.

A little after midnight on November 28, 2014, hundreds of Black Lives Matter protesters filled the streets of downtown Chicago. The demonstration was one of many that erupted in cities nationwide soon after a Missouri grand jury failed to indict a Ferguson, Missouri, police officer for the shooting death of Michael Brown that August. As the protesters marched, a police vehicle crept behind them. The black SUV emblazoned with "City of Chicago Emergency Management" appeared to have two 360-degree cameras sprouting from its roof and a command center in the back.

Whenever the vehicle drove by, protesters reported that their phones stopped working.

A week later, audio of a police radio dispatch from the protest was released online. In the recording, an officer alerts a department intelligence analyst about of one of the protest organizers. "One of the girls here… she's been on her phone a lot," the officer says. "You guys picking up any information? Where they're going, possibly?