Federal and state Homeland Security grants allow local law enforcement agencies to surveil American citizens with technology more commonly found in war zones and foreign espionage operations. At least two Texas communities along the U.S.-Mexico border have purchased a product called "TraffiCatch," which collects the unique wireless and Bluetooth signals emitted by nearly all modern electronics to identify devices and track their movements. The product is also listed in a federal supply catalog run by the U.S. government's General Services Administration, which negotiates prices and contracts for federal agencies. Combining license plate information with data collected from wireless signals is the kind of surveillance the U.S. military and intelligence agencies have long used, with devices mounted in vehicles, on drones or carried by hand to pinpoint the location of cell phones and other electronic devices. Their usage was once classified and deployed in places like Afghanistan and Iraq. Today, similar devices are showing up in the streets of American cities. The Supreme Court has said that attaching a GPS tracking device to a car or getting historical location data from a cell carrier requires a search warrant. However, law enforcement has found ways around these prohibitions. Increasingly, as people walk around with headphones, fitness wearables and other devices ... their data can be linked to a car, even after they have ditched the car. Courts have not definitively grappled with the question: Under what circumstances can law enforcement passively capture ambient signal information and use it as a tracking tool?