The FBI used advanced technology on airplanes to monitor protesters in Baltimore in the week after Freddie Gray's funeral, documents released under the Freedom of Information Act show.
The agency confirmed months ago that it flew over the city in April, but it had provided few details. Files released Friday by the American Civil Liberties Union reveal more about the surveillance operation.
FBI aircraft made 10 flights and logged more than 36 hours — mostly at night. A member of the Baltimore Police Department joined federal agents on some of the flights. They captured video and infrared images, some of which have been retained by agents.
The FBI has flown similar missions around the country, often without public notice. The flights can provide coverage of a large area more cheaply and less obtrusively than helicopters.
Authorities see such flights as a useful way to monitor a city during unrest, but privacy advocates have expressed concern.
"There undoubtedly are times when aerial surveillance is an appropriate law enforcement tool for public safety or investigative purposes," Nathan Freed Wessler, an ACLU attorney, wrote in a blog post Friday.
"But it is essential that accurate information about such surveillance be available to the public, and that strict rules be in place to protect against unjustified mass surveillance or warrantless collection of private information."
FBI director James Comey has defended the Baltimore flights.
"If there is tremendous turbulence in a community, it's useful to everybody — civilians and law enforcement — to have a view of what's going on," Comey told the House Judiciary Committee last week. "Where are the fires in this community? Where are people gathering? Where do people need help?"
The FBI has taken the position that it doesn't need a warrant to conduct aerial surveillance because it is watching people in public and does not focus on individuals.
While law enforcement agencies do not typically need permission from a judge to watch people in public, the Supreme Court has grappled in recent years with the implications of increasingly sophisticated surveillance technology.
GPS bugs, for example, allow police to follow a target in much the same way as they could by tailing them. In 2012, the high court ruled that placing a bug on vehicle amounted to a search, and some justices wrote that using a tracker for an extended period of time violated a person's privacy.
Wessler wrote that in some cases, infrared cameras can effectively see through walls. If they do, the courts have said, police must have a warrant.
Gray, 25, died in April after suffering a severe spinal cord injury in police custody. His death inspired days of protests against police brutality. On the day of his funeral, the city erupted in riots, arson and looting.
The FBI took to the skies two days later. The city had been placed under curfew, and National Guard troops had been deployed to help police maintain order. But the city was having difficulty monitoring the streets, FBI officials wrote in an internal memo, and so it asked the agency to provide aerial surveillance.
The FBI flew over the city from April 29 to May 3. On May 1, State's Attorney Marilyn J. Mosby announced charges against six police officers involved in Gray's arrest and transport, calming the city.
The flights came to light when local aviation enthusiasts noticed strange aircraft in the area and looked up their flight paths on a public radar website. The site showed several planes, including small Cessnas and a jet, making unusual looping flights around parts of the city, and revealed tail numbers identifying some of the aircraft.
Federal Aviation Administration documents released to The Baltimore Sun show extensive modifications to one plane, including exhaust silencers, mapping systems and infrared cameras.
The FBI said the planes were equipped with cameras that can use lasers to capture images in near-total darkness. On at least one of the planes, that equipment was hooked up to a system that could overlay street names and other geographic information on a live video feed, according to FAA records.
Electronics diagrams in the records include what appears to be a code name for one of the tools: "FBI Hawk Owl Project."
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Privacy groups have raised concerns that the aircraft could carry cell site simulators — a technology known by the trade name Stingray, which lets law enforcement track the location of cellphones and harvest information about calls.
The Wall Street Journal has reported that the U.S. Marshals Service has put the devices on aircraft.
An FBI spokesman said the flights over Baltimore were not using any such gear.
"FBI aircraft are not equipped or used for bulk collection activities or mass surveillance," Christopher M. Allen said. "FBI surveillance flights in support of the Baltimore Police Department in April were not collecting cellphone data."
An Associated Press investigation uncovered a web of front companies that the FBI uses to register at least 50 planes with aviation authorities while obscuring their operator. Reporters tracked 100 flights over 11 states and the District of Columbia, with operations above several major cities.