Riot police Baltimore
Riot police line the streets of Baltimore in the wake of riots. Getty Images

In the latest chilling sign of an increasingly sophisticated sci-fi level of surveillance by authorities, city police and federal officials used surveillance flights to track Baltimore protests and neighborhoods, witnesses discovered

The "eyes in the sky" involving at least two planes cruised over Baltimore three consecutive nights after the city erupted in riots in response to the death of Freddie Gray, 25, in police custody. Six officers have been charged in his death.

Residents communicating via Reddit and Twitter tracked the flight paths of a Cessna 560 Citation V, owner unknown, and a Cessna 182T Skylane plane registered to NG Research in Bristow, Vermont, on April 30 and the first two days of May. "Persistent surveillance from low-flying aircraft is happening now," tweeted one of the civilian trackers.

The FBI confirmed that it provided the spy planes to the Baltimore police to provide "aerial imagery of possible criminal activity," reports the Washington Post.

The American Civil Liberties Union is challenging the government's authority to launch the flights. Planes outfitted with the latest surveillance systems can track far larger areas than police helicopters and stay overhead longer, which raises fears of civil liberties violations. The aircraft can capture detailed images of residents and their activities whether they're involved in a crime or not, gathering huge amounts of information about citizens under no suspicion of criminal activity.

"A lot of these technologies sweep very, very broadly, and, at a minimum, the public should have a right to know what's going on," Jay Stanley of the ACLU told the Post. The organisation has already filed a Freedom of Information Act Request for all details of the flights.

Flight records from the Web tracking site Flightradar24 reveal the aircraft flew precise formations — always after dark— over the section of West Baltimore where the rioting occurred. The planes used infrared technology to follow people on ground, a source told the Post.

One of the first to start tracking the flights was Pete Cimbolic, a former ACLU staff member, who was alerted by resident Benjamin Shayne in a Twitter post. Shayne operates the site which streams live audio from police department radios in the city.

It's unclear what connection one plane's owner, NG Research, might have to the US government, but a Reddit post reported a sighting of the plane over Langley, West Virginia, near CIA headquarters.