Maryland U.S. Attorney Rod J. Rosenstein will argue the government’s case at the 4th Circuit that law enforcement should be able to obtain cellphone location data without a warrant. (Michel du Cille/The Washington Post)

When high-powered rifle shots shattered a Florida judge’s living-room window and glass door, federal agents narrowed the list of suspects by pulling their cellphone records. After two California college students were fatally shot in a car parked close to campus, cellphone data put the suspects near the crime scene at the time of the shooting.

Investigators in Maryland pulled seven months of phone records to track the movements of two men later convicted in armed robberies around Baltimore.

Law enforcement officials have long relied on location details gleaned from cellphone towers as a powerful tool for tracing steps of suspects, particularly in the early stages of investigations.

But civil liberties groups and privacy advocates are increasingly challenging the practice. They are concerned that police and federal agents can too easily tap vast caches of information about people’s movements through devices most Americans carry in their pockets — trackings that could show how often someone goes to a doctor’s office, to a casino or to church.

Legislators on Capitol Hill have proposed updates to federal statutes, but no standardized rules exist for scooping up location data. Instead, local and federal investigators rely on a patchwork of state laws and inconsistent court rulings.

A federal appeals court on Wednesday will be the latest front in the legal debate, in the case that involves the Baltimore robberies.

The issue before a full panel of the U.S. Court of Appeals for the 4th Circuit, which has jurisdiction over Maryland and Virginia, is whether investigators need a search warrant before they can track suspects’ long-term movements through their cellphones.

A three-judge panel of that court, based in Richmond, ruled in August that accessing the location information without a warrant for an “extended period” is unconstitutional because it allows law enforcement to trace a person’s comings and goings across public and private spaces.

But two other federal appellate courts — in Florida and New Orleans — concluded that warrants are not necessary.

If the full 4th Circuit upholds its panel’s 2-to-1 decision, there would be a clear split with the other federal appeals courts. That type of divide often attracts the attention of the Supreme Court, which has already expressed concern about the effect of long-term surveillance by law enforcement on individual privacy.

“There is a sense that judges and others have that electronic evidence [collection] presents a risk to privacy that other forms of evidence don’t. That anxiety is a thread through these court decisions,” said former federal prosecutor Jason M. Weinstein, who oversaw cybercrime and organized-crime enforcement in the Justice Department’s criminal division.

Texting, calling, and checking email or the weather from a cellphone generally involves connecting with the closest communications tower. Wireless providers log and retain records showing which tower a phone used at the beginning and end of every call, and increasingly, for texts and data connections.

In the Baltimore case being heard Wednesday by as many as 16 judges on the court, police obtained 221 days of data from the wireless provider of robbery suspect Aaron Graham. The 30,000 location points generated for his phone enabled authorities to map his whereabouts before and after two of six robberies and to corroborate evidence during a 2012 trial. Graham and co-conspirator Eric Jordan were sentenced to decades in prison.

The American Civil Liberties Union, which has signed on in support of the pair’s appeal, took the records analysis even further and showed that authorities also could have connected the dots to place Graham at the office of his pregnant wife’s obstetrician.

“The more of this data you have, the more you are able to peer into the patterns of somebody’s life,” said Nathan Wessler, an ACLU lawyer who wrote a brief in support of the defendants that was joined by other groups, including the Electronic Frontier Foundation and the Center for Democracy and Technology.

“We’re talking private locations and private information. We want the police to have to jump through a few hoops,” Wessler said. “That’s what protects us.”

The debate in court this week is over how high to set the hurdle.

Until recently, law enforcement officials had little trouble obtaining the cell-tower records with a court order, which requires them to provide less rigorous information than they would need to get a search warrant.

The distinction is a vital one to law enforcement.

A court order clears the way to the cell-tower records that early in an investigation are the building blocks needed to reach the gold standard of probable cause required for more-intrusive searches.

Without that early access through court orders, former prosecutors say, it will be more difficult to zero in on suspects and rule out others.

“There’s no question there will be crimes that are not solved,” said Weinstein, who also prosecuted violent crimes in Baltimore. “This is a stand the government has to make.”

In a series of recent Supreme Court decisions, the court signaled that digital devices are different when it comes to Fourth Amendment protections against unreasonable search and seizure because of the vast amounts of personal information stored on phones and tablets.

As a result, Justice Department policy now requires a warrant in most cases before agents install GPS-tracking devices on vehicles. Investigators generally must also obtain a warrant to operate cellphone-data collectors, called cell-site simulators or Stingrays, that are facing legal challenges, including in the nation’s capital.

The department generally distinguishes between real-time, pinpoint surveillance and the type of historical business records obtained in the Maryland armed-robberies case. The more precise the information and intrusive the search, the stronger the privacy interests and the higher the legal standard, according to Richard W. Downing, acting deputy assistant attorney general, who testified this month before the House Committee on Oversight and Government Reform.

Law enforcement officials say the cellphone records in the Baltimore case are no different from landline telephone records or banking transactions that authorities have long been able to obtain without a warrant because the documents are business or “third party” records.

“When the government obtains historical cell-site records, it is not monitoring ongoing events; instead it is obtaining information concerning past events that was previously collected by a third party,” according to Maryland prosecutors whose argument won support from the dissenting judge on the initial panel.

In a sign of the importance of the data to investigators, the government’s case will be argued by Maryland U.S. Attorney Rod J. Rosenstein.

No matter which side prevails in the 4th Circuit, the final ruling could still be bad news for Graham and Jordan, the men in the Baltimore robbery case.

Even the initial panel of judges that sided with the defendants on the cell-tracking issue ultimately upheld their convictions, saying police had acted in good faith under the current rules.

The panel’s finding that warrants were needed applied to law enforcement requests going forward for the cell-tower data.