The FBI used low-flying planes to capture hours of video footage and other intelligence during protests over police brutality in Baltimore, according to documents obtained by the ACLU. The documents, released under the Freedom of Information Act, show that the federal agency helped local law enforcement watch protests between April 29th and May 3rd by making 10 flights over the area, comprising a total of 36.2 hours. According to the ACLU, at least half the flights provided police with high-resolution video, which was delivered later in May.

FBI director James Comey has previously confirmed that the agency flew planes over protests in Baltimore — as well as in Ferguson, MO — in order to support local police in case of riots or other emergencies. Earlier this year, reports showed that the FBI has operated surveillance planes over 30 cities through a series of shell companies that conceal its involvement. In addition to recording video, the planes are reportedly equipped with other sensors, including infrared cameras and "dirtboxes" that can capture information from cellphones below them. According to the documents, some of the Baltimore flights conducted "electronic surveillance," but any more specific details were redacted.

In this case, the planes were gathering footage from protests over the death of Freddie Gray, who died of a spinal cord injury while in a police van. In late April — shortly before the first flight recorded here — violence had broken out during one of the protests, and the mayor issued a city-wide curfew. In one of the documents, the agency says that "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," requiring the FBI to step in with more resources.

The FBI has said that recording aerial footage in public locations doesn't require a warrant, and that the aircraft are only used in specific cases — "We don't fly planes around America looking down to see if somebody might be doing something wrong," said Comey last week. But the ACLU argues that as surveillance technology becomes more advanced, things like infrared sensors can start capturing sensitive information that wouldn't be evident from simple video. "There undoubtedly are times when aerial surveillance is an appropriate law enforcement tool," writes staff attorney Nathan Freed Wessler. But as technology moves forward, he says, "we must ensure that old legal rules from the pre-digital era are not blindly applied to newer and more powerful forms of digital surveillance."