Civil liberties groups are concerned that the FBI may have violated the privacy rights of West Baltimore residents after it was revealed that the bureau secretly piloted two unmarked aircraft over the city’s skies during public unrest after the death of a black man in police custody, The Washington Post reported Tuesday.

Twitter users spotted and identified the two aircraft — one small jet and one small propeller plane — circling above the area where city residents rallied in support of Freddie Gray and protested, some violently, against alleged police brutality.

The FBI on Wednesday confirmed to the Post that it did provide aircraft to city police for the purpose of “providing aerial imagery of possible criminal activity.”

Gray, 25, was arrested on April 12 and suffered a spinal injury while in police custody. His death one week later sparked protests across the city. While most were peaceful, some protesters turned violent, clashing with police and setting public and private property on fire. Maryland Gov. Lawrence Hogan responded by declaring a state of emergency and deploying the Maryland National Guard.

Baltimore police helicopters hovered day and night over West Baltimore during the protests. At night, they swept over the streets with searchlights as officers commanded residents through megaphones to obey a 10 p.m. curfew or face arrest.

The unrest largely subsided on May 1, after state attorney Marilyn Mosby announced that six police officers involved in Gray’s arrest would face criminal charges ranging from assault to second-degree murder. Nevertheless, the malaise exposed deep fissures between the city’s police department and black community.

With trust in city police weakened, Baltimore Mayor Stephanie Rawlings-Blake on Wednesday asked the Department of Justice, which includes the FBI, to investigate whether the city’s police department has a pattern of abuse or discrimination.

On the same day, the American Civil Liberties Union filed a Freedom of Information Act request with the FBI and Federal Aviation Administration to determine what the aircraft were doing patrolling Baltimore’s skies.

The request asks for “all records regarding surveillance or monitoring equipment carried on such flights, including its capabilities and description of the data gathered by it.”

In a statement on Wednesday, ACLU senior policy analyst Jay Stanley said the group was concerned that law enforcement was using the aircraft to violate residents’ Fourth Amendment rights.

“Today planes can carry new surveillance technologies, like cellphone trackers and high resolution cameras that can follow the movements of many people at once,” he said. “These are not the kinds of things that law enforcement should be using in secret. The public needs to know about the government’s use of these powerful technologies to ensure that people’s rights are protected.”

In recent years, police departments across the U.S. have added new, more capable spy equipment to their arsenals, including tools that can track people’s locations through cellphone data. Civil liberties groups worry such technology puts at risk the privacy of people who are not suspected of any crimes.

The FBI told the Post that it did not employ such surveillance technology during the Baltimore protests, but the bureau declined to comment on the planes.

Electronic Frontier Foundation senior counsel Jennifer Lynch told Al Jazeera that phone data isn’t the only thing protesters have to worry about. Powerful cameras circling above can record individuals’ faces and activities.

In 2012, she said, the Los Angeles County Sheriff’s Department flew a spy plane over the Compton area of Los Angeles, observing the commission of crimes. Most of them amounted to petty theft, The Los Angeles Times reported. But the flights went on in secret for nine days.

The people of Compton, a historically impoverished black neighborhood, didn’t know about the flights, just as the residents of Baltimore didn’t know they were being monitored by the FBI last month.

“When the government conducts surveillance on people without telling people what kind of information they’re collecting, the power balance shifts,” Lynch said. “Instead of being in hands of people, it’s completely in the hands of government.”

According to Greg Nojeim, a lawyer focusing on privacy issues at the Center for Democracy and Technology, that shift could stifle free speech and political activity.

“Police monitoring can chill free speech because people don’t speak as freely when they believe that law enforcement or other government entities are recording what they say,” he said.