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Baltimore police have used secret cellphone interceptors more than 4,300 times

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Correction April 21st 11:21AM ET: The Baltimore Sun has retracted the key figure that made this story notable. The Sun originally reported that Stingrays had been used more than 25,000 times, but has since adjusted the figure to 4,300, as previously reported.

The Baltimore Police Department is starting to come clean about its use of cell phone signal interceptors — commonly known as Stingrays — and the numbers are alarming. According to recent court testimony reported by The Baltimore Sun, the city's police have used Stingray devices with a court order more than 4,300 times. That number doesn't include any emergency uses of the device, which would have proceeded without a court order.

Stingrays work by mimicking cell towers, sniffing out phones and triangulating their location once the phones start to transmit. Because the Stingray is masquerading as a conventional cell tower, it disrupts any cell traffic in the vicinity of the target as well, often cutting out service for other devices in the area. A number of projects have attempted to build Stingray detectors, but those devices have often been accused of excessive false positives, effectively overstating the spread of Stingrays. In one recent test in Washington, DC, a detector found 18 different Stingray-like disturbances over less than two days, suggesting that either the detector is faulty or the devices are far more widespread than previously believed.

If the numbers are true, they suggest aggressive and sustained use of a technology that is still largely undisclosed by civilian police departments. Police departments still routinely drop cases rather than reveal the use of Stingrays in court, most recently in a robbery case in St. Louis. The devices have been defended as measures for emergency-use only — often in the case of a cell phone-triggered bomb. But using the device nine times a day would mean it has become a routine part of police work in Baltimore, and potentially other US cities as well.