This piece was originally posted on MSNBC.com

It all started with a simple Twitter query from a Baltimore man: “Anyone know who has been flying the light plane in circles above the city for the last few nights?” This was several days after the death of Freddie Gray in police custody, and the subsequent riots and protests. Pete Cimbolic, an aviation buff and former ACLU employee who saw the tweet, was able to identify the plane, a Cessna, and get its registration information using online aviation resources. He also found that a separate aircraft, a small jet, was flying perfect circles over West Baltimore for hours on end. He alerted the ACLU of Maryland, which (having its hands full with many pressing civil liberties violations in Baltimore) forwarded it to those of us who work on surveillance at the national ACLU.

Today we filed a Freedom of Information request with the FBI, DEA, and U.S. Marshals Service seeking information about these mysterious flights. We've also filed a FOIA request with the FAA to get the aircrafts' flight plans.

Why the curiosity over a simple aircraft circling overhead? After all, as everyone who was in Baltimore at the time knows, police helicopters were a constant presence in the skies over the affected neighborhoods. And in a situation where there are protests in the streets, and some causing property damage, the police certainly have a legitimate need to keep a handle on what's happening.

The answer is, these are not your parents' surveillance aircraft. Today there are powerful new surveillance technologies that use aircraft to collect mass information about whole populations, potentially reaching far beyond what the police might need to manage unrest. One such technology, known informally as "Dirtboxes," has been used to sweep up identifying information about tens of thousands of cell phones in a single flight. These devices (the ground-based versions of which are known as Stingrays) are also technologically capable of stealing data off of phones, though we don't know whether they've been used that way yet.

Another mass surveillance technique that uses fixed-wing aircraft is known as "Wide-Area Surveillance." This involves the installation of super-high, gigapixel resolution cameras on planes, which are then used to monitor entire cities. Every moving pedestrian and vehicle can be tracked: the beginning and end everyone’s journeys, and the route taken in between. This gives the authorities the power to press "rewind" on anybody's movements, and learn a lot of intrusive things about how they live their life. Among the companies offering this technology are major defense contractors working for the Pentagon and an Ohio company called Persistent Surveillance Systems, which is trying to sell it to local police departments.

These are just the technologies that we know about so far.

Investigating the flights over Balitmore, The Washington Post reporter was told by an unnamed "government official" that the flights were apparently carried out by the FBI at the request of local law enforcement, and that they were using infrared cameras of some kind "to monitor movements of people in the vicinity." That could mean a range of different things.

It's possible the craft weren't doing much more than basic aerial spotting for the police. But they may also have been using some of these very new and very powerful technologies. We as a society are going to need to grapple with what kinds of rules are needed for such devices to strike the right balance between the needs of law enforcement and the need to protect the privacy of individuals and prevent our country from becoming a surveillance society.

In a democracy, these are value judgments that should be made by the public. But too often we have seen government officials arrogating unto themselves the power to make these judgments – for example by deploying technologies such as cell phone trackers and license plate scanners without even informing communities, let alone securing their permission. Through our FOIA requests and other efforts, we will continue pushing to ensure that the American public at least knows what's going on.