In 2011 federal prosecutors were working with magistrate judges in the Northern District to resolve concerns about the government’s use of sophisticated surveillance technology known as a stingray to track people using their cellphone signals.

The problem, as described in an email from Criminal Division chief Miranda Kane, involved agents’ use of stingray devices, also known as WIT or triggerfish, without obtaining specific permission from the court. Such devices, which simulate a cell tower, can be placed in a van and driven anywhere in order to pinpoint the location of wireless devices like cellphones or broadband Internet cards.

Now, Kane’s May 2011 email, along with two responses, have emerged at the center of a controversy over the Justice Department’s use of stingrays to track suspects, what magistrates are being told, and what legal standard applies to the warrants. The issue, like the recent U.S. Supreme Court decision on GPS tracking, lies at the intersection of privacy and evolving technology.

Civil liberties advocates contend the emails obtained last month by the ACLU and submitted in a federal criminal case in Arizona show that federal agents in the Bay Area and perhaps elsewhere routinely used stingrays without informing the court.

"It’s a smoking gun," said Hanni Fakhoury of the Electronic Frontier Foundation, one of the organizations questioning the Justice Department’s surveillance in the Arizona case. "It validates the exact argument we raised in our amicus brief that prosecutors weren’t telling judges they were using stingrays."

Stingray devices raise privacy concerns in part because they are indiscriminate and draw information from all devices on the same network in a given area, Fakhoury said.

A spokesman for U.S. Attorney Melinda Haag declined to comment. However, Justice Department lawyers in the Arizona case have defended the employment of a stingray in the investigation of Daniel Rigmaiden, a pro se defendant charged with perpetrating a multimillion-dollar tax fraud by filing false tax returns, including some in the names of dead people.

Rigmaiden’s case is one of the first to test the constitutionality of stingray technology, which is relatively new and lacks clear legal precedent. Though the prosecution is in Arizona, key aspects of the investigation unfolded in the Northern District. Then-U.S. Magistrate Judge Richard Seeborg issued the order under which agents employed the stingray device. That device was used to track Rigmaiden’s broadband Internet card to a unit in a Santa Clara apartment complex.

The Northern District emails, written roughly three years after the Rigmaiden investigation, were released March 22 under a Freedom of Information Act request by the ACLU.

In her email, Kane urged prosecutors to be "consistent and forthright" regarding plans to use a stingray when they sought court approval for pen register devices, the more traditional means for tracking incoming and outgoing cellphone calls.

"As some of you may be aware, our office has been working closely with the magistrate judges in an effort to address their collective concerns regarding whether a pen register is sufficient to authorize the use of law enforcement’s WIT technology," Kane stated. She added: "It has recently come to my attention that many agents are still using WIT technology in the field although the pen register application does not make that explicit."

Kane, who rejoined the U.S. attorney’s office in late 2010 as part of Haag’s management team, directed prosecutors to seek a supervisor’s approval before requesting court authorization for a pen register and to review their requests from the prior six months to determine whether a stingray had been used.

Karen Beausey, then chief of narcotics prosecutions, responded that prosecutors should consider whether the initial intended purpose of the pen register was to use a stingray or whether agents later made that decision. (Beausey has since moved to the U.S. attorney’s office in Boston.)

"And just to be super clear," wrote Assistant U.S. Attorney Kyle Waldinger, "the agents may not use the term ‘WIT’ (or ‘WITT’) but rather may be using the term ‘Triggerfish’ or the term ‘Stingray,’ so please make sure that the agents know what you are referring to."

The emails raise questions about the practice of prosecutors and agents prior to 2011 and during the investigation of Rigmaiden, San Francisco ACLU attorney Linda Lye said in an interview.

"The email confirmed that what we thought in Rigmaiden, that the government was not being forthright, was part of a larger pattern that went on for at least three years," Lye said. "To the extent corrective actions have been taken that’s obviously a necessary and welcome development."

U.S. District Judge David Campbell in Phoenix is considering the Northern District emails in connection with a motion to suppress evidence in Rigmaiden’s case. The ACLU and EFF have joined Rigmaiden in arguing the use of stingray in his case violated the Fourth Amendment because Seeborg, now a U.S. district judge in San Francisco, was not specifically informed that stingray technology would be used. Seeborg issued an order in July 2008 directing Verizon Wireless to assist agents in ascertaining the physical location of a Verizon broadband access card that had been linked to the fraudulent transmissions.

Jennifer Granick, director of civil liberties at the Stanford Center for Internet and Society, said it would be troubling if prosecutors did not inform Seeborg that stingray technology would be used to locate Rigmaiden’s computer.

"There’s a big difference between getting data from the provider and using this kind of technology on your own, and it doesn’t appear in the Rigmaiden case that the investigators spelled out for the magistrate what they were going to be doing," Granick said. "That’s a very serious problem."

Vanessa Blum is a reporter for The Recorder, a Legal affiliate based in San Francisco.