In days after Baltimore burned with unrest over Freddie Gray’s death in police custody, two FBI surveillance planes circled low above the city. But they weren’t using normal cameras to capture activities on the ground.

Instead, the planes used a system with sophisticated thermal-imaging and night-vision capabilities, according to newly released FBI and Federal Aviation Administration documents obtained by the American Civil Liberties Union through a Freedom of Information Act request. And the law enforcement agency held on to the videos it took during the flights.

The new details are raising concerns among privacy advocates who warn that advances in surveillance technology are being deployed domestically without public debate, based on legal arguments that do not account for new developments in technology.

“This is a dynamic we see again and again when it comes to advances in surveillance,” Nathan Wessler, an ACLU staff attorney, told The Washington Post. “By the time details leak out, programs are firmly entrenched, and it’s all but impossible to roll them back — and very hard to put in place restrictions and oversight.”

The planes used in the Baltimore flights, reported on by The Post shortly after they were seen, were hidden behind shell companies. A later investigation by the Associated Press revealed that the FBI was operating a small fleet of similar spy planes across the country, all using front companies.

The government argues that such secrecy is necessary to protect the aircraft and pilots — and to avoid alerting the subjects of ongoing investigations to the flights.

“Contrary to some recent media reporting, the FBI’s aviation program is not classified. Some of our aircraft are registered covertly because overt registration would put our aircraft and operations at risk of compromise,” the FBI said in a statement in the wake of the AP report.

“The FBI routinely uses aviation assets in support of predicated investigations targeting specific individuals and, when requested and appropriate, in support of state and local law enforcement,” it said.

In the case of the Baltimore flights, the agency’s role was to monitor the general movements of the crowds to preserve public safety, according to an FBI official with knowledge of the situation who spoke on the condition of anonymity because he was not authorized to comment on the topic. In an internal memo obtained by the ACLU and shared with The Post in advance of its publication, the FBI stated that “large scale demonstrations and protests” were being scheduled and the “potential for large scale violence and riots” as justification for the flights.

The new documents show that at least one of the two planes flown as part of that mission was equipped with a FLIR Talon unit — a thermal-imaging system with infrared cameras and a “laser pointer/illuminator” that can identify targets for those using night-vision devices “while remaining invisible to others,” according to promotional material.

Between the two planes, the agency flew 10 surveillance flights over Baltimore from April 29 through May 3, adding up to more than 36 hours in the air, according to the documents.

In an e-mailed statement, FBI spokesman Christopher Allen told The Post that the agency’s planes are “not equipped or used for bulk collection activities or mass surveillance.” However, Allen confirmed that the flights did use the plane’s thermal-imaging capabilities. “Each flight produced infrared and day color, full-motion FLIR video evidence, which is maintained in accordance with record retention policies,” he said.

The agency did not clarify what those retention policies are or how long this type of footage is retained. But it ruled out the use of a type of cellular surveillance technology known an IMSI catcher or cell-site simulator, which sucks up data by tricking cellphones into connecting to it by pretending to be the nearest cell tower.

“FBI surveillance flights in support of the Baltimore Police Department in April were not collecting cellphone data, nor were they equipped with cell-site simulators,” Allen said. In September, the Department of Justice issued a new policy requiring law enforcement agencies to obtain a probable cause warrant to use the cellular tracking technology in most cases.

The agency did not seek or obtain warrants for the Baltimore flights.

In a section of the FBI’s Domestic Investigations and Operations Guide obtained by the ACLU, the agency says that “aerial surveillance conducted from navigable airspace” does not invoke Fourth Amendment protections requiring a warrant because of Supreme Court precedents about reasonable expectations of privacy in public spaces.

The ACLU takes issue with that interpretation, arguing that the use of tools such as infrared imaging may change that equation. For instance, the group notes that the Supreme Court has already weighed in on thermal-imaging tech. In the 2001 decision Kyllo v. United States, the justices held that the use of a thermal imager to “see” details from inside a home without a warrant amounted to an unlawful search.

Some modern aerial surveillance systems can use infrared technology to track people at night, even through tree cover or some structures. If monitored over time, privacy experts say, data collected from such setups could allow for the identification of individuals even if the system’s cameras are not powerful enough to clearly show features such as faces or license plates, because people’s routines typically vary little from day to day.

In testimony before the House Judiciary Committee last week, FBI Director James B. Comey appeared to say that the agency does not use capabilities that could reveal the details of activities inside buildings as part of its surveillance flights.

“I suppose if you are putting technology on an FBI aircraft that had Fourth Amendment implications, that is that it was reaching someone’s communications or looking within a dwelling or something like that, it would have Fourth Amendment implications,” he said. “That’s not what we use the aircraft for.”

In light of the new ACLU documents, Comey’s response may raise more questions than answers for some civil liberties advocates who now carefully parse answers from senior officials about surveillance programs because of misleading answers in the past. Is he signalling that the thermal-imaging technology loaded onto his agency’s planes isn’t powerful enough to detect activities inside a home? Or perhaps it’s merely that the agency does not use the technology for the express purpose of digitally peering inside homes?

The FBI did not initially address questions posed by The Post about the level of detail captured by the system used in the Baltimore flights or if the agency considered the implications of the Kyllo precedent as it added thermal-imaging technology to its aviation program. In a statement shared after the story was published, Allen said that the infrared cameras on FBI aircraft “are not able to identify specific heat signatures through a solid object.”

But even with new information about the Baltimore flights, privacy advocates say they are just starting to piece together the details of the FBI’s aerial surveillance program.

“These documents provide insight into 10 flights by two FBI planes over the course of five days in one city. But we know the FBI has many of these planes, flying over many of these cities, many days a year,” Wessler said. “A lot of questions are unanswered.”

This story has been updated with further comment from the FBI about the capabilities of the thermal imaging systems in use by its aircraft.