New York police are so determined to catch the vandals who replaced the American flags atop the Brooklyn Bridge that they’re using an investigative technique known as “tower dumping” to examine all of the cell phone calls made near the bridge around the time the flags were replaced. The controversial method has been used to solve violent crimes in the past, but civil liberty advocates suggest the NYPD, embarrassed by the public flag display, is bringing out unnecessarily invasive surveillance technology to catch the perps.

New Yorkers woke up July 22 and looked up at the top of the iconic Brooklyn Bridge to find not the red, white and blue American flag waving above the 131-year-old landmark, but rather two bleached white flags. Exactly what was behind the stunt remains unclear, with guesses ranging from a prank to Banksy-esque street art to an act of terrorism, but city officials were quickly looking for an explanation to the question of how exactly how five young adults could climb both of the 276-foot towers without being detected.

“Needless to say, no matter what the motive was, it is a matter of concern,” NYPD Commissioner Bill Bratton told reporters. “I am not particularly happy about the event.”

A New York Daily News story published two days after the incident revealed police found DNA on the bridge they hope will identify those responsible. But anonymous police sources also briefly mentioned the NYPD has been “analyzing data from the two nearest cell phone towers” to find a lead on the case.

Tower dump investigations occur when police convince a judge to grant a court order compelling the operator of the cell tower (or towers) in question to provide a log of all the calls made and/or received within a certain vicinity within a certain window of time. In the Brooklyn Bridge case, for instance, police have likely used a surveillance video showing the flags being switched around 3:30 a.m. Tuesday to ask that all cell transmissions in the area between 3:10 and 3:50 a.m., for instance.

(The NYPD did not return repeated requests for comment on this story.)

The only problem with this technique, civil liberties advocates say, is the court order required to obtain those records has a lower burden of proof than probable cause, which is required for a search warrant, even though a search warrant only applies to one person whereas a tower dump order could ensnare thousands or, in New York City, possibly hundreds of thousands.

A federal judge told International Business Times even without the surveillance footage, the police “can probably get the order” though it would have to be “fairly targeted in terms of time.”

The NYPD is also using social media data, video, facial recognition technology and approximately 18,000 license plate pictures in trying to solve the case.

Tower dump orders have quietly become a point of contention among judges, phone companies and police investigators, who have debated whether the method violates the Fourth Amendment, which prohibits unreasonable searches and seizures while requiring warrants to be supported by probable cause.

“Some companies have said they require a warrant; others have said they do not,” Hanni Fakhoury, a staff attorney who focuses on criminal law and privacy at the Electronic Frontier Foundation, told IBTimes. “There’s no litigation on this. No one has challenged this because it’s very new. The government would argue they don’t need a warrant because it’s targeted information.”

Another possible reason tower dumping has yet to be challenged is because it was successfully used in 2006 to apprehend the killer of Imette St. Guillen, a popular New York City graduate student. St. Guillen’s death at the hands of nightclub bouncer Darryl Littlejohn made national headlines and shook New York City, but it also had prosecutors thankful that they were able to use data from a nearby cell tower to prove Littlejohn traveled from his home to the place where St. Guillen’s body was found.

The EFF and other organizations are in favor of using tower dumps to apprehend suspected murderers, robbers and rapists. Yet Fakhoury said they draw the line when police try to obtain an order that ensnares so many people for the sake of a nonviolent crime.

“Do we really want the police doing a tower dump on every single person near the Brooklyn Bridge on a Tuesday?” he asked. “That’s stupid, that’s an abuse and it’s something we should think about legislating. Putting up a white flag on the Brooklyn Bridge is not enough of an excuse to violate the privacy of tens of thousands of people on a given day.”

A growing number of judges have agreed. Vast surveillance requests had a better chance of being approved in the days when cell phone technology was still new, and before it was revealed U.S. intelligence agencies conduct widespread surveillance on American phone metadata. Uniformity has become increasingly difficult to come by though, with some magistrates granting orders that others would declare unconstitutional.

“State and federal courts have barely addressed cell tower dumps,” wrote magistrate-turned-Texas Tech law professor Brian Owsley in a landmark 2013 law paper. “However, the actions by most of the largest cell phone providers, as well as personal experience and conversations with other magistrate judges, strongly suggest ‘that it has become a relatively routine investigative technique’ for law enforcement officials.”

The debate is complicated even further by questions on the accuracy of tower dump data. An individual’s location could easily be misinterpreted if a phone connects to a cell tower that has a strong signal rather than the closest cell tower. It’s this lingering uncertainty, and the potential for abuse, that has civil liberty groups encouraging police to only use tower data in the most egregious circumstances.  

“We think tower dumps are extremely invasive,” Fakhoury said of the EFF’s position. “They should only be used in certain situations after a judge found probable cause. Using this for the Brooklyn Bridge is overkill beyond imagination.”