A federal judge in Oakland, California has ruled against the suppression of evidence derived from warrantless use of a cell-site simulator. The simulator, a device often referred to as a stingray, was used to locate the lead defendant in an ongoing attempted murder case.

In the 39-page ruling, US District Judge Phyllis Hamilton notably found that the use of stingray to find a man named Purvis Ellis was a "search" under the Fourth Amendment—and therefore required a warrant. However, in this case, the judge also agreed with the government’s assertion that there were exigent circumstances, along with the "good faith exception" to the warrant requirement. In other words, use of the stingray was wholly justified.

"Cell phone users have an expectation of privacy in their cell phone location in real time and that society is prepared to recognize that expectation as reasonable," Judge Hamilton wrote, citing an important Supreme Court decision from 1967 known as United States v. Katz.

But because Ellis was believed to be involved in another shooting that happened one day earlier on January 20, 2013, the judge felt there were exigent circumstances.

"Though Ellis was not known to be the shooter, he was believed to be a suspect in possession of firearms," Judge Hamilton continued. "The need to prevent escape by a suspect presented exigent circumstances here."

"Exigent circumstances" is the idea in American criminal procedure that law enforcement can search or seize persons or things if there are imminent circumstances where bodily harm or injury is in process, evidence is being destroyed, or a suspect is in flight. In such situations, a warrant is not required.

Warrants required since 2015 anyway

Thursday’s court order could provide a new incentive for a guilty plea for Ellis, who was located in an East Oakland apartment building by the FBI and the Oakland Police Department several hours after the January 21, 2013 shooting of a non-uniformed OPD officer. Three other men were charged in the case, although one of them pleaded guilty earlier this year. No trial date has been set.

Ellis’ attorney, Martha Boersch , did not immediately respond to Ars’ request for comment.

As Ars has been reporting for years, Ellis has provided rare insight into stingrays are used in practice to find suspects and the seeming lengths the government is willing to go to keep usage quiet. The surveillance tool has come under increasing scrutiny by lawmakers and activists in recent years. Since this case began, both the Department of Justice, which oversees the FBI, and the State of California now require a warrant when a stingray is used in most circumstances.

Riana Pfefferkorn, a lawyer affiliated with the Stanford Center for Internet and Society, said that the judge’s ruling was "careful," but she noted that it may not specifically matter, given that both state and federal policy has changed since Ellis and his co-defendants were arrested in 2013.

"This is resolving something that happened over four years ago where on a going forward basis it may be a moot point," she told Ars.

When this case first began unfolding in federal court four years ago, prosecutors insisted that only one stingray was used. It turned out that there had been two—one first by the OPD, followed by another belonging to the FBI.

The entire issue as to what level of privacy a person can expect over their location information, and what hurdles law enforcement must jump through to obtain it (through a stingray or other means), are still up for discussion.

A related case, United States v. Carpenter, is pending before the Supreme Court. In Carpenter, the court is being asked whether law enforcement needed a warrant to obtain over 120 days of cell-site location information (CSLI), or whether a lesser standard was sufficient.

"The court's search analysis is based heavily on that one district's CSLI cases, which conflict with other circuits' rulings on CLSI and are going to be decided soon by the Supreme Court anyway," Orin Kerr, a law professor at George Washington University, told Ars. "Given that the constitutional analysis is largely based on issues the Supreme Court is about to revisit, it's not obvious that Ellis will have much influence on its own."