Confirming long-held suspicions by privacy advocates, newly obtained U.S. Department of Justice (DOJ) documents reveal that the secretive police surveillance devices known as Stingrays are capable of recording phone conversations and spying on mobile device users within the vicinity of a target.

Stingrays mimic cell phone towers to trick nearby mobile devices into connecting with them in order to pinpoint and tracked users' locations, a surveillance tactic that has reportedly been employed during recent anti-police brutality protests.

Now, the ACLU's investigation has turned up damning evidence that the surveillance technology has an even greater reach than the DOJ has previously admitted.

Internal DOJ documents (pdf) obtained through a Freedom of Information Act (FOIA) request filed two years prior show that Stingrays are "capable of intercepting the contents of communications" of cell phones that connect to the faux tower signal, including recording outgoing numbers dialed and possibly intercepting conversations and texts by wirelessly changing a device's firmware. The memos caution agents not to collect such information, "as that would entail surveillance on the calling activity of all persons in the vicinity of the subject."

That means Stingrays "spy on innocent bystanders," ACLU senior staff attorney Linda Lye wrote in a blog post Wednesday.

In fact, the agency itself acknowledges as much in the documents, writing that the use of cell phone tracking technology "is an issue of some controversy," although it does not elaborate further.

The documents also explain how Stingrays are able to intercept conversations by "using a suspect's cell phone as a bug."

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"If the cellular telephone is used to make or receive a call, the screen of the digital analyzer/cell site simulator/triggerfish would include the cellular telephone number (MIN), the call's incoming or outgoing status, the telephone number dialed, the cellular telephone's ESN, the date, time, and duration of the call, and the cell site number/sector (location of the cellular telephone when the call was connected)," the documents state.

Also included in the documents is a form instructing agents how to request a court order to use Stingrays in criminal investigations. But as Lye pointed out in her blog post, the form uses "opaque language" to describe the technology, notably omitting any mention that the devices could ensnare innocent bystanders.

"DOJ realized it was important to educate its own prosecutors that cell site simulators can conduct 'surveillance on the calling activity of all persons in the vicinity.' It should have said so in plain English to courts as well," Lye wrote.

In September, the DOJ announced new rules for the use of Stingrays which require law enforcement agencies to obtain warrants by probable cause before deploying the surveillance tools in any criminal investigation. But the memos reveal that the DOJ grants wide exceptions in cases with vaguely termed "exceptional" circumstances, such as a hack on a government computer or "activity characteristic of organized crime."

"While such crimes are potentially serious, they simply do not justify bypassing the ordinary legal processes that were designed to balance the government’s need to investigate crimes with the public’s right to a government that abides by the law," Lye writes.

At least 13 federal departments are known to be in possession of Stingrays, including non-law enforcement agencies like the Internal Revenue Service (IRS).

Moreover, the DOJ's new rules don't apply to local police departments, which have been known to use the devices even as first-resort measures to investigate minor crimes.

As ACLU staff attorney Nathan Freed Wessler pointed out at the time, the limited regulations leave open wide loopholes that allow "warrantless use of Stingrays in undefined 'exceptional circumstances' while permitting retention of innocent bystander data for up to 30 days in certain cases."