For reasons that remain unclear, the Internal Revenue Service (IRS) has been in possession of the controversial cell phone spying devices known as Stingrays, the Guardian exclusively reported on Monday.

Invoices obtained following a request under the Freedom of Information Act show purchases made in 2009 and 2012 by the federal tax agency with Harris Corporation, one of a number of companies that manufacture the devices.

The ACLU, which has called for stricter oversight of the technology, describes Stingrays—also known as "cell site simulators" or "IMSI catchers"—as "invasive cell phone surveillance devices that mimic cell phone towers and send out signals to trick cell phones in the area into transmitting their locations and identifying information. When used to track a suspect's cell phone, they also gather information about the phones of countless bystanders who happen to be nearby."

Stingrays require only a low-level court order called a PEN register to grant permission for their use.

"Immense secrecy has so far surrounded these devices, but a picture is slowly emerging which shows widespread use," write Guardian reporters Nicky Woolf and William Green. "Various revelations by the American Civil Liberties Union and news outlets including the Guardian had shown that at least 12 federal agencies are already known to have these devices, including the National Security Agency and the Federal Bureau of Investigation. The IRS makes 13."

The devices are also used by local and in some cases state police departments, across at least 20 states, the Guardian adds. The ACLU provides a map here.

Just last week, Homeland Security Assistant Secretary Seth M. Stodder explained to a U.S. House subcommittee that the Secret Service, too, can employ Stingrays without a warrant if there's believed to be a nonspecific threat to the president or another protected person.

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While no one from the agency responded to a request for comment, a former IRS employee suggested to the Guardian that such widespread adoption across law enforcement agencies may in fact explain why the IRS would find itself utilizing such technology:

[Mark Matthews, a former deputy commissioner for services and enforcement at the agency who now works for the law firm Caplin and Drysdale] said the IRS on its own usually uses gentler investigation tactics. But increasingly, investigating agents from the agency are brought on board for joint operations with the FBI and other agencies when the latter need financial expertise to look at, for example, money laundering from drug organizations. From these joint operations, he said, "the IRS had moved to drug work and had learned a lot of aggressive techniques in the money laundering and drug world, and these bad habits were leaking over into the tax world, which was supposed to be their real mission."

Nate Wessler, a staff attorney with the speech, privacy, and technology project at the ACLU, told the Guardian: "The info showing that they are using Stingrays is generally consistent with the kinds of investigative tactics that they are engaging in, and it shows the wide proliferation of this very invasive surveillance technology."

"It's used by dozens, perhaps hundreds, of local law enforcement, used by the usual suspects at the federal level," Wessler added, "and if the IRS is using it, it shows just how far these devices have spread."