PHOENIX, Arizona – The government's use of a secret spy tool was on trial on Thursday in a showdown between an accused identity thief and more than a dozen federal lawyers and law enforcement agents who were fighting to ensure that evidence obtained via a location-tracking tool would be admissible in court.

At issue is whether law enforcement agents invaded Daniel David Rigmaiden's privacy in 2008 when they used a so-called stingray to track his location through a Verizon Wireless air card that he used to connect his computer to the internet. Also at issue is whether a warrant the government obtained from a judge covered the use of the stingray and whether the government made it sufficiently clear to the judge how the technology it planned to use worked.

Over the course of a three-hour hearing in the U.S. District Court in Arizona, Rigmaiden, 31, asserted that the warrant the government obtained only authorized Verizon Wireless to provide agents with data about the air card but did not authorize agents to use the invasive stingray device. He also asserted that Verizon Wireless "reprogrammed" his air card to make it interact with the FBI's stingray, something that he says was outside the bounds of the judge's order.

Rigmaiden and civil liberties groups who have filed amicus briefs in the case also maintain that the government failed its duty to disclose to the judge who issued the warrant that the device they planned to use not only collected data from the target of an investigation but from anyone else in the vicinity who was using an air card or other wireless communication device.

Linda Lye, staff attorney for the American Civil Liberties Union of Northern California, told the judge on Thursday that this was the equivalent of rummaging through ten or twelve apartments to find the correct one where the defendant resided, something that would never be allowed under a normal warranted search.

By withholding information about the stingray from the judge and providing only "scant information" about the data they planned to collect, the FBI had "failed to live up to its duty of candor.... The government should have been clear about what information it wanted to obtain and what information it was going to obtain in using the technology," she said.

The ACLU recently uncovered emails that show a pattern of agents routinely withholding information from judges about their use of stingrays in applying for warrants for electronic surveillance.

The Rigmaiden case is shining a spotlight on the secretive technology, generically known as a stingray or IMSI catcher, that allows law enforcement agents to spoof a legitimate cell tower in order to trick nearby mobile phones and other wireless communication devices into connecting to the stingray instead of a phone carrier's tower.

When devices connect, stingrays can see and record their unique ID numbers and traffic data, as well as information that points to the device’s location. To prevent detection by suspects, the stingray is supposed to send the data along to a real tower so that traffic continues to flow.

By gathering the wireless device’s signal strength from various locations, authorities can pinpoint where the device is being used with much more precision than they can get through data obtained from a mobile network provider’s fixed tower location.

Although there are stingray devices that can capture and record the content of phone calls and text messages, the U.S. government has insisted in court documents for the Rigmaiden case that the stingray used in this case could only capture the equivalent of header information — such as the phone or account number assigned to the aircard as well as dialing, routing and address information involved in the communication. As such, the government has maintained that the device is the equivalent of devices designed to capture routing and header data on e-mail and other internet communications, and therefore does not require a search warrant.

The device, however, doesn’t just capture information related to a targeted phone. It captures data from “all wireless devices in the immediate area of the FBI device that subscribe to a particular provider” according to government documents -— including data of innocent people who are not the target of the investigation. FBI policy requires agents to then purge collateral data collected by the surveillance tool.

The StingRay II, made by the firm Harris. The stingray name is also a generic term for other cell tower devices that operate similarly to this one. Image: U.S. Patent and Trademark Office

Rigmaiden was arrested in May 2008 on charges that he was the ringleader of an identify theft gang that stole more than $4 million in refunds from the IRS by filing fraudulent tax returns. Rigmaiden and others are accused of using numerous fake IDs to open internet and phone accounts and using more than 175 different IP addresses around the United States to file the fake returns, which were often filed in bulk as if through an automated process.

Rigmaiden, along with Ransom Marion Carter, III, who is still at large, was charged with 35 counts of wire fraud, 35 counts of identify theft, one count of unauthorized computer access and two counts of mail fraud.

Investigators used the stingray to trace an air card to the Domicilio Apartments complex in Santa Clara, California, that Rigmaiden was renting under an assumed identity. Court documents indicate that the surveillance tool led investigators “to the general proximity of defendant’s usage of the aircard,” allowing authorities to then narrow the air card’s location to three or four apartments, and finally to unit 1122 in the complex.

Rigmaiden maintains that in order for the stingray to be able to collect location data from his air card, Verizon Wireless had to write data to the air card consisting of "identifying information for the FBI's emulated cell sites" as well as make configuration changes that would cause the air card to recognize the FBI's emulated cell tower as an authorized tower for providing service and cause the air card to attempt connections to the emulated tower prior to attempting connections with actual Verizon Wireless towers.

"The FBI technical agents needed Verizon Wireless to write data to the aircard in this manner because the aircard's properly configured Preferred Roaming List prevented it from accessing rogue, unauthorized cell sites... such as cell site emulators used by the FBI," Rigmaiden wrote in a court document.

For years, the U.S. government asserted that the use of stingray devices did not violate Fourth Amendment rights, and Americans didn’t have a legitimate expectation of privacy for data sent from their wireless devices to a cell tower, even if another device secretly intercepted that communication to the tower.

But the government changed its stance in the Rigmaiden case in 2011 after the defendant argued that using the device to locate his wireless air card inside an apartment constituted a search under the Fourth Amendment and therefore required a valid search warrant, which Rigmaiden asserted authorities didn’t have.

After U.S. District Judge David Campbell indicated he’d seek more information about the device and how it worked, prosecutors backed down – likely to avoid scrutiny of their technology – and conceded that in this case alone its use could be considered a search, but that in general the devices did not violate American’s privacy.

Prosecutors then argued that the FBI's use of the stingray against Rigmaiden was indeed authorized by a court order and warrant. They pointed to a warrant that agents obtained to get near real-time tracking information from Verizon Wireless and said that was sufficient to cover the stingray as well. A separate tracking warrant, prosecutors said, wasn’t necessary for the fake tower.

A U.S. attorney argued in court on Thursday that although the warrant used in the case didn't specify that it was for a tracking device, the affidavit seeking the warrant did, and this was sufficient. He acknowledged, however, that the affidavit didn't describe what device the FBI planned to use or how it worked.

The ACLU's Lye called the government's assertions a "post hoc re-characterization" of the warrant and said the government was simply "laboring" to find any document it could, in order to say it had obtained a warrant for the stingray.

When asked by Judge Campbell if it was important for the judge who issued the warrant to know that the location-tracking tool they planned to use was designed to collect data from other nearby devices, Assistant U.S. Attorney Fred Battista said that "the use of these devices is a very common practice," implying that the technology should not be new to judges. Battista also said that the stingray collected only a "small amount of data" from other devices in the area. The data had been destroyed afterward and there was no evidence that any third party suffered harm as a result of the collection, he said.

He also asserted that in all the cases in which stingray's had been used, there was no indication that a judge had ever disallowed the use of the tool because it could pick up data from other devices not belonging to a suspect.

Lye disputed this, however, and cited two cases where judges brought up concerns about the fact that the devices vacuum up any data in their path. The matter of issuing the warrants in those cases, however, was ultimately overtaken by other matters, making the data collection a moot point.

Rigmaiden is seeking to have evidence obtained through the stingray thrown out on grounds that what he has characterized as "warrantless surveillance" violated his privacy.

Government prosecutors adopted a circular argument on Thursday to counter this by saying that even though the use of the stingray in this case constituted a search under the Fourth Amendment and required a warrant – thereby carrying an expectation of privacy – Rigmaiden himself didn't have any expectation of privacy since he had rented the apartment under false pretenses by using a stolen identity to obtain the lease and had also obtained his Verizon Wireless account using another stolen identity.

"This is a very unique case," Battista told the judge. He urged the court not to be so swayed by Rigmaiden's eloquent assertions of his privacy rights that it forgot to take into account the fact that he cared little about the privacy rights of the victims he allegedly exploited when he stole their identities.

"A person who acts as the defendant has acted should not have any expectation of privacy," Battista said.

The government is fighting so hard for its use of the stingray in this case, because if Rigmaiden succeeds in his motion to suppress evidence obtained through the stingray, it would significantly weaken the government's case against him, since it could also force the government to throw out evidence obtained in a search of his apartment and a separate storage space where investigators seized computer hard drives. Information obtained through the stingray and the air car was at least used as part of the basis for obtaining warrants for those other searches.

"They did get a separate search warrant to search the apartment, but the question is did they have independent probable cause in that search warrant application separate from the stingray search?" Lye says.

Lye notes that this case is much different from other location tracking cases – such as the GPS tracking case the Supreme Court decided last year. Other cases have focused on whether or not the government should obtain a warrant to use a location-tracking device or to obtain location data from a third-party provider. In Rigmaiden's case, however, the government has already conceded it needed a warrant.

"The question here is whether this odd order the government obtained was a warrant, and that gets to the question about the government duty to be candid with the courts about the technology," she says.

Lye urged the judge to suppress evidence obtained through the use of the stingray in Rigmaiden's case "to ensure that the government [will be forthcoming] with judges in the future."

Judge Campbell is expected to rule on the motion to suppress within a few weeks.

Rigmaiden's trial is scheduled to begin in May.