A federal appeals court in New Orleans last week took up an issue of increasing national importance, exploring the clash between privacy rights and law enforcement’s exploitation of technology to investigate and prosecute crimes.

The U.S. Justice Department went to court Oct. 2 to try to convince a panel of the 5th U.S. Circuit Court of Appeals to overturn a magistrate judge’s ruling that blocked the government from obtaining historical cell tower records from two phone companies.

Prosecutors in Texas want to review the tower records — showing the placement and receipt of mobile phone calls — in three cases that involve, separately, drugs, alien smuggling and gang-related extortion. The cases are ongoing, Nathan Judish, a senior DOJ lawyer who specializes in computer crime, told the appeals court.

The government, Judish argued, should not be required to obtain a warrant, which requires a higher standard than a court order, to review historical tower data from a mobile carrier. Those records, he said, are business documents created and kept at the discretion of individual phone companies. Subscribers have no control over the private data, Judish told Senior Judge Thomas Reavley, who heard the case with judges James Dennis and Edith Clement.

Judish compared phone companies to prospective witnesses in possession of information about a potential crime. In the investigatory stage of a criminal case, he said, agents want to review small scraps of information to bolster actual charges.

“Law enforcement fundamentally needs building blocks in which to develop probable cause for investigations, and cell site information is one of those building blocks which is critical to law enforcement today,” Judish said in court.

Susan Freiwald of the University of San Francisco School of Law, who writes about cyberlaw and information privacy, argued in the 5th Circuit in support of a search warrant requirement under the Fourth Amendment. The government, Freiwald said, should not be allowed to police itself over its surveillance authority. Freiwald said DOJ should be required to present sufficient information to a neutral magistrate judge to first obtain a warrant.

“Without the warrant requirement, agents are engaging in wide-scale and mass surveillance of innocent people in ways that infringe upon not only on privacy rights but First Amendment rights of association and expression,” Freiwald told the panel judges.

“We need the warrant requirement to rein in the scope and scale of this mass surveillance to ensure that the law enforcement has an adequate justification for this surveillance.”

Hanni Fakhoury of the Electronic Frontier Foundation, arguing against the DOJ position, told the appeals court that “while law enforcement should be able to have access to this information in order to solve crimes and keep us safe, it should only be able to do so if it obtains a search warrant supported by probable cause.”

Clement and Fakhoury discussed at one point the extent to which a mobile phone user voluntarily provides call location information to a phone company every time a call is made or answered.

“Nobody makes you have a cell phone,” Clement said. “If you’re up to no good, maybe you ought not to have a cell phone.” The judge noted that cell phone subscribers knowingly sign contract documents that contain disclosure policy and practices.

A person should not have to choose between Fourth Amendment privacy protections and enjoying the conveniences of a modern world, Fakhoury said in court. There’s a growing understanding “that cell phones are becoming a way of life, a ubiquitous way of life,” he said.

Judish, in his closing remarks, said cell tower information isn’t comparable to the warrantless real-time Global Positioning System tracking that was involved in a case the U.S. Supreme Court decided earlier this year. In that case, the high court ruled that the warrantless installation of a tracking device violated a Washington-area man’s privacy rights.

DOJ lawyers argued in court papers that cell tower data isn’t as precise as GPS information. A court order, which doesn’t require a showing of probable cause, should be sufficient to obtain tower data, Judish said.

“Why is it harder to get a warrant?” Clement asked Judish. “Why is it more burdensome for the government to have to show probable cause?”

Investigators, Judish argued, will use a warrant to obtain evidence of a crime. Cell tower information itself, he said, may not reveal an actual crime. He urged the court to allow the government to “develop a fuller picture” through the warrantless review of cell tower data.

The appeals court didn’t immediately rule after the hour-long hearing.