The FBI deployed at least 10 flights of surveillance planes equipped with advanced aerial surveillance technology, including infrared and night-vision cameras, to monitor the Baltimore riots earlier this year, according to government documents obtained by the American Civil Liberties Union.

The flights, totaling more than 36 hours and involving at least two planes, occurred over Baltimore from April 29 to May 3, showed the flight logs provided to the ACLU under the Freedom of Information Act. The ACLU, a civil rights group, released the logs to Reuters and other news organizations on Friday.

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Half of the flights carried Baltimore police, in addition to FBI officials, and a majority of them occurred at night over Baltimore during several days of civil disorder that followed the death of Freddie Gray while in police custody.

The FBI has previously acknowledged that surveillance flights occurred.

In congressional testimony last week, FBI Director James Comey did not elaborate in detail on how surveillance flights are conducted or approved.

But he acknowledged that planes were also used at the request of local authorities during protests last year in Ferguson, Missouri, after the shooting death of Michael Brown, an unarmed black man, by a white police officer.

The documents released by the ACLU “fill some of the gaps that we and the public have had about the government’s use of surveillance technology on these planes,” said Nathan Freed Wessler, a staff attorney at the ACLU, which is concerned about privacy and profiling issues raised by aerial surveillance.

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The Baltimore Police Department referred questions about the flights to the FBI.

In a statement to Reuters, FBI spokesman Christopher Allen said each Baltimore flight “produced infrared and day color, full-motion FLIR (forward-looking infrared) video evidence which is maintained in accordance with record retention policies.”

An FBI memo dated May 1 described the support provided at the request of the Baltimore authorities.

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“The potential for large scale violence and riots throughout the week presents a significant challenge for the Baltimore Police Department for airborne surveillance and observation,” the memo noted in its justification of the flights.

One plane used was a Cessna propeller plane that, according to documents from the Federal Aviation Administration, possessed an infrared camera mount and a multi-sensor camera system manufactured by FLIR Systems, an Oregon-based company.

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The widespread and largely secretive use of aerial surveillance has drawn attention from Congress, where lawmakers in recent months have pushed the FBI to be more forthcoming about the legal authority and technical scope of such flights.

(Reporting by Dustin Volz; Editing by Kevin Drawbaugh Alan Crosby)