Sometimes federal agents nab their suspect by following investigative leads or a money trail. But it was a trail of cellular signals gathered with a device known as a stingray that in 2008 led FBI agents to the defendant in $3 million tax fraud scheme by tracking his wireless aircard to an apartment unit in Santa Clara.

A stingray, also known by the nickname triggerfish, mimics a cell tower and can be used to pinpoint the location of wireless phones and aircards. At the same time, according to civil liberties advocates, it sucks in information from all other gadgets in its radius.

In one of the first rulings to analyze the legal framework for such surveillance, U.S. District Judge David Campbell of Arizona on Wednesday upheld the FBI’s use of a stingray to track down Daniel Rigmaiden, finding no violation of his Fourth Amendment rights "given the unique circumstances of this case".

Campbell denied a defense motion to suppress evidence, concluding agents acted in good faith and properly obtained a warrant based on probable cause prior to employing a stingray. The Phoenix-based judge, who is presiding over the prosecution brought in the District of Arizona, also determined law enforcement’s use of a stingray to investigate Rigmaiden was not a "severe intrusion" under the Fourth Amendment.

Moreover, he reasoned that Rigmaiden, a pro se defendant whose legal cause has been supported by the American Civil Liberties Union and the Electronic Frontier Foundation, had no legitimate expectation of privacy in the wireless aircard hunted by FBI agents after its IP address was linked to more than 1,200 allegedly fraudulent tax returns.

"Defendant has not shown that his Fourth Amendment rights were violated or, if a violation did occur, that suppression is the appropriate remedy," Campbell wrote.

The detailed 52-page ruling in U.S. v. Rigmaiden, 08-814, is part of developing case law on the standards for a relatively new and sophisticated surveillance technology. A 2012 decision in the Southern District of Texas rejected a request to use a similar device based on inadequacies in the government’s application to the court. That ruling noted "scant case law addressing the equipment."

Since the Rigmaiden case was investigated in Northern California, the litigation drew focus to the U.S. attorney’s office here. The 2008 warrant was signed by then-U.S. Magistrate Judge Richard Seeborg.

Attorney Linda Lye of the ACLU in San Francisco called the Rigmaiden decision "highly consequential and very troubling."

Lye had argued for suppressing the stingray-gathered evidence because federal prosecutors did not explicitly disclose plans to use the device when they sought court approval. The application submitted to Seeborg sought authorization for a "mobile tracking device" to monitor a Verizon aircard and did not explain it would capture signals from all other cell phones in the area, Lye said.

"The magistrate was the one who should have been giving the government guidance," Lye said. "The fact that it was new technology means the government should err on the side of providing more, not less information."

Campbell sided with government lawyers, who argued FBI agents were using a relatively new technology and lacked legal precedent regarding the proper form for the warrant.

"There is no precedent suggesting that the agent was required to include in his warrant application technical details about the operation of the mobile tracking device," Campbell wrote. "Technical information about the device was not material to the probable cause determination, nor is there a basis for concluding that such details were omitted in bad faith."

At the time, Seeborg, now a district judge, had expressly found probable cause to believe that the use and monitoring of a mobile tracking device would lead to evidence of various crimes, Campbell noted, distinguishing it from the Texas case where a magistrate denied use of a stingray under an order with a lesser evidentiary standard.

One of the key legal fights was whether federal prosecutors dealt honestly with Seeborg with regard to the intended use of a stingray. Internal emails from the Northern District U.S. attorney’s office obtained by the ACLU and submitted to the court indicated that agents had used stingrays without obtaining specific permission from the court.

Addressing the 2011 emails in a footnote, Campbell said they showed "an evolving understanding about the use of technology, with prosecutors in the Northern District of California attempting to be responsive to concerns expressed by magistrate judges."

The emails were sent years after the Rigmaiden investigation, Campbell wrote, and did not demonstrate any deficiency in the submission by prosecutors or agents in that case.

A spokesman for the U.S. Attorney Melinda Haag declined to comment.

Lye said Campbell’s order leaves open several important questions about the use of stingrays. The government stipulated — only for the purposes of this case — that a stingray constitutes a search, requiring a warrant. However, prosecutors are not bound by that position in other cases.

"I suspect these issues will get to the Ninth Circuit," Lye reflected, "but on a much more delayed schedule than we hoped."

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