We might finally have a constitutional challenge to secretive stingray surveillance, after men accused of shooting a California police officer have contested the warrantless use of a cell phone tracking device to locate them.

Reveal's Ali Winston reports:

In the case stemming from the 2013 shooting of Officer Karsseboom, a motion filed in February by defense attorneys Christopher Cannon and Matthew Laws alleged that Oakland police had tracked down their clients by locating one of their cellphones with a StingRay. Accompanying documents, including redacted transcripts of police communications, show that gang task force officers conducted an electronic sweep of the area and located the accused men and Karsseboom’s stolen guns by locking in on the transmissions from a target cellphone.

The use of a StingRay to find the location of the target cellphone was confirmed in an April motion by Assistant U.S. Attorney Joseph M. Alioto Jr.: “The United States confirms that a cell-site simulator was used in this case to obtain the general location of an armed suspect at large. … Even if the use of a cell-site simulator now requires a warrant, its use in this case was under exigent circumstances. The Defendant was thought to be armed and involved in the shooting of an Oakland police officer several hours earlier.”

The Northern California U.S. attorney’s office did not respond to requests for comment on this case or the use of evidence obtained by cell-site simulators.

“I think they have to suppress what they obtained and the fruit of what they obtained,” Cannon said in an interview with Reveal.

In other cases where criminal defense attorneys have challenged warrantless stingray spying, prosecutors have agreed to lenient pleas or dropped prosecutions altogether to avoid an open court battle about the technology and its use. That's because the FBI has forced police departments that want to buy stingrays into signing non-disclosure agreements, requiring prosecutors and police to confer with federal agents before discussing the technology publicly, even in court. Thus far none of those challenges have survived the FBI's scrutiny.

But this California case deals with people who are accused of shooting a cop. Law enforcement isn't likely to want to go easy on these defendants simply to keep a stingray challenge out of court. Therefore we might soon see the first court ruling on the constitutionality of warrantless stingray spying.

Stay tuned to find out what happens, and read more about the case.