Civil rights groups are raising serious constitutional questions about the Justice Department’s use of dragnet technology onboard aircraft to collect data from suspects’ cell phones, as reported by the Wall Street Journal Thursday.

The program, run by the U.S. Marshals Service, uses small aircraft equipped with high-tech devices that mimic cell towers, tricking suspects’ cell phones into connecting with them instead of legitimate towers. The devices, called dirtboxes, can then grab certain data from the tricked phones, most notably their location. The aircraft involved operate from five U.S. metropolitan areas and have together a flying range covering most of the country’s population, the Journal reported.

The program is designed to target suspects in law enforcement investigations. However, the nature of the technology means that devices in a certain range of the aircraft are fooled into connecting to the dirtbox, potentially giving law enforcement access to identifying data and general location information about hundreds or thousands of innocent Americans with each flight. Because that access comes without probable cause, civil liberties groups say, the program could be a violation of the Fourth Amendment.

“These devices are sweeping up information about the cell phones of thousands of completely innocent bystanders. That looks a whole lot like the kind of dragnet search that the framers of the Fourth Amendment abhorred,” said American Civil Liberties Union attorney Nathan Wessler.

The Justice Department said it could not confirm or deny the existence of the program. But a department official said that all federal investigations are consistent with federal law and are subject to court approval. That official also said the Marshals Service does not maintain any databases of cell phone information — meaning the program could possibly only be used to track the whereabouts of suspects on a case-by-case basis and that it’s vastly different in nature from the kinds of sweeping government surveillance programs first revealed by Edward Snowden.

Still, is the Justice Department’s airborne dragnet program legal? The answer is “maybe.”

Federal authorities have employed similar tools in the past. The Federal Bureau of Investigation is known to use a surveillance tool called a “stingray,” a portable transceiver that tricks cell phones within a certain area into relaying their locations, not unlike the equipment onboard the Marshals’ aircraft. A government vehicle with a stingray can net hundreds of nearby cell phones’ approximate locations just by driving through a typical neighborhood. The government has said it doesn’t need a probable cause warrant to use stingrays because investigators don’t collect the content of phone calls, just the locations of those phones. Government officials, meanwhile, have said they get court approval to use the devices.

Much of the government’s warrantless use of stingray-style technology hinges on a 1979 Supreme Court decision titled Smith v. Maryland. Smith involved law enforcement’s use of a device called a pen register that, when attached to a suspect’s phone line, recorded the numbers of outgoing calls, but not the calls themselves. The Smith decision upheld the warrantless use of such devices because the suspect’s phone company would record the same data picked up by the pen register, and therefore the suspect had no reasonable expectation of privacy when it came to that information. Currently, the law requires a court to approve the use of a pen register, but investigators only have to show that the device’s use is “relevant to an ongoing criminal investigation,” a much weaker standard than a probable cause warrant requires.

Hanni Fakhoury, an attorney at the pro-privacy Electronic Frontier Foundation, says the Department of Justice could use the Smith precedent as legal justification for the airborne dirtbox program. However, Fakhoury also highlighted a key problem with that argument: Location. Pen registers aren’t intended to pick up location data beyond an area code, whereas the airborne dirtboxes can track a person down to a single building. Many courts, he said, have expressed that location data deserves greater constitutional protection than is afforded to other kinds of information.

However, to get back to the Smith decision, wireless carriers do store your location history for several months to several years, information they obtain by keeping a record of the cell towers to which your device connects as you move from place to place. That could mean Americans don’t have a reasonable expectation of privacy over their location data and the Smith precedent applies, making the DoJ’s aerial surveillance program legal. Still, that would be a matter for the courts to decide.

“There are a lot of tricky questions whether a stingray or dirtbox operated by the government directly is a pen register, or the Fourth Amendment concerns dismissed by the Supreme Court 35 years ago in Smith v. Maryland are more applicable here,” Fakhoury said.

Contact us at letters@time.com.