A few weeks ago, the U.S. Department of Homeland Security issued a warning about “unauthorized” use of devices that mimic cell phone towers to track phones in D.C.

It provided info that the so-called stingray devices could be used by foreign powers, but it remained unclear just how many were being used, and who was using them.

Taking up the case, NBC Washington set out to find where they might be in use. Working with mobile security expert Aaron Turner, the news station’s I-Team detected 40 potential readings that indicated activity in a few hours of monitoring.

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