The Department of Homeland Security (DHS) says it has observed “anomalous activity” consistent with the use of so-called stingrays.
They could be used by foreign spies or criminals, although the DHS said it did not know who was using them.
It added that such devices pose a “growing risk”.
Stingrays, a brand name for a type of International Mobile Subscriber Identity catcher (IMSI), are mobile phone surveillance devices that mimic mobile phone towers.
The size of a briefcase, the devices send out signals to trick mobile phones into transmitting their location and identifying information.
As well as tracking the mobile phone of a suspect, the devices also gather information about phones of bystanders who are nearby.
It is believed to be the first time the US government has acknowledged the use of rogue spying devices in Washington.