For up to 10 hours a day, a Cessna propeller plane circled the city of Baltimore, secretly monitoring about 600,000 people, capturing their movements and transmitting the data to private security analysts.

Operated by a firm called Persistent Surveillance Systems, the small plane circled 8,500 feet above ground snapping wide-angle images of citizens represented as one-pixel dots on computer screens below.

“One pixel per person allows me to follow a dot, which usually goes a block or so, jumps into a car and drives off,” says Ross McNutt, whose company assisted Baltimore police in a trial program earlier this year. “We follow the people and vehicles to and from crime scenes.”

That program, first reported by Bloomberg in August, has since ended. Police spokesman T.J. Smith says the department last operated planes on Oct. 15, monitoring a major naval event and the Baltimore marathon. Since then, “police departments from around the world” have contacted the city to express interest in the aerial program and determine its effectiveness, says Smith, who declined to specify which ones.

The program highlights growing interest among police departments in tools that often elicit criticism when made public. An astronautical engineer who trained at the Massachusetts Institute of Technology, Mr. McNutt says his company has conducted operations in about a dozen US cities – including Philadelphia and Los Angeles – that closely resembled the Baltimore trial.

“No one wants to be first … they get yelled and screamed at,” says McNutt, who says he welcomes public debate around his surveillance technology. “It’s, ‘Oh my God, Big Brother’s watching me.’ Well, I hate to tell you, there’s cameras all over the place. All we’re doing is helping make them effective.”

WATCH: Special report on surveillance in Baltimore

More than a thousand cameras on stoplights, buildings and poles surveil Baltimore passersby throughout the city streets. Since 2005, police have paired privately owned cameras with city ones, weaving together a vast surveillance net in which little remains private.

But its surveillance goes well beyond conventional measures. Baltimore uses technology known as “Stingrays” – devices that impersonate cellphone towers to capture and track calls – license-plate readers, and facial recognition to monitor citizens. In August, three privacy groups filed a Federal Communications Commission (FCC) complaint alleging the city’s above-average use of Stingrays disproportionately affected minority communities and disrupted cell service.

While conventional surveillance tools such as wiretapping and cameras have established usage guidelines, these new technologies exist in a sort of “Wild West,” largely unhindered by regulations and oversight.

McNutt says his technology, which is bound by a strict internal privacy policy even as many of his clients don’t have one, was adapted from a tool he built for the US Army. In 2006, at the peak of the Iraq war, the Pentagon asked McNutt to help cut down deadly roadside bombs. His solution? Angel Fire, a wide-area aerial surveillance system that could monitor the roads in real-time.

“It invariably happens that technology developed for military applications gradually percolates out to domestic policing,” says Julian Sanchez, a senior privacy fellow at the libertarian Cato Institute. “We talk about police militarization usually thinking about … SWAT-style gear, but there’s a parallel phenomenon on the surveillance side.”

But Smith, the Baltimore police spokesman, says as criminals adapted to existing technologies, police needed to become more “creative.” Since news of the aerial program broke, he says the department has received an “unprecedented” number of calls from citizens hoping to solve old crimes. He had not, however, heard a single complaint from “communities most affected by this violence.”

And there is some indication it may be working. After its bloodiest year ever in 2015, Baltimore’s homicides have dropped 7 percent, from 311 to 290. Smith said the week when Baltimore’s surveillance program was revealed, it had only one daytime shooting, down from an average of more than five in weeks before. And, he added, the program has led to at least one murder prosecution and helped investigate multiple nonfatal shootings.

“We have a responsibility as a public safety agency to do everything legally possible to capture the people who committed these crimes,” Smith says. “People who are trying to harm us get more and more creative every day.”

Is surveillance working?

Jay Stanley, a privacy expert at the American Civil Liberties Union, says new technologies such as aerial surveillance have potential on a “case-by-case basis,” but need public debate and oversight. This fall, the ACLU launched legislative efforts in 11 US cities to promote “community control over police surveillance” by mandating elected officials – not police departments – decide when to adopt new surveillance technology.

“The primary checks and balances of local democratic decision-making bodies is the power of the purse,” Mr. Stanley says. “We’re seeing that system being short-circuited because police departments are getting outside grants. … So police departments can get this bag of money dropped in their laps ... and they don’t have to go to the city council to get permission.”

In addition, Stanley says many new surveillance tools lack strong reporting metrics, making it difficult to determine their effectiveness. McNutt’s company, for example, has devised ambitious goals, such as cutting crime by 30 percent, but most remain untestable since his programs operate in secret.

In Baltimore, Smith says his department would wait for an upcoming report to determine if it should continue aerial surveillance.

The report, set for release next month, is being compiled by the Police Foundation, a nonprofit that facilitated a private grant from the Laura and John Arnold Foundation to fund the aerial surveillance program. While the Police Foundation declined to share specific details from the report, president Jim Bueermann said the program in Baltimore was “probably” not set up with a “rigorous scientific method” and had “data issues.”

“Across the landscape of policing there are very few performance measurements in place,” he says. “It just has not historically been something that policymakers have demanded and few police chiefs and sheriffs have spent enough time on trying to understand what works and what doesn’t.”

When asked about the lack of a “rigorous scientific method” for analyzing the surveillance program, Smith said he had “no knowledge of that.”

Quickly evolving technology

Across the country, police are utilizing and repurposing existing technology to monitor citizens and fight crime – without much data to back it up. In Oakland, Calif., for example, police received federal funds to build a Domain Awareness Center capable of analyzing live video in conjunction with license-plate scanners and gunshot detectors. When citizens discovered the center in 2013, they were shocked.

“Not a single person in my circle knew this project existed,” says Brian Hofer, an Oakland-based privacy advocate. “How has this multimillion dollar surveillance project gone this far with no public scrutiny? You can’t regulate something if you don’t even know it exists.”

Left unchecked, the hub would have bolstered police surveillance by merging live video and license plate readers with newer thermal imaging and body movement software. In the future, documents showed, it could have included facial recognition to allow for real-time video analysis.

Mr. Hofer and other privacy advocates successfully scaled back development of the center in 2014 and he now chairs a standing privacy committee in Oakland.

Alvaro Bedoya, executive director of the Center on Privacy and Technology at Georgetown University, says unchecked use of facial recognition – as proposed in Oakland – threatens democracy. In a recent report, the center called for greater transparency over enforcement agencies that access photos of more than 117 million Americans often without legislative approval.

“Does it look like America when you walk outside for a protest and you can have your face photographed, scanned, and identified in secret by your government?” Mr. Bedoya says. “Security and safety is not the end-all be-all of how we run our government. Freedom also has a say in that.”

In the future, Bedoya says, surveillance will shift from devices to people. Law enforcement will track immutable characteristics like iris structures, behavioral patterns and voice recognition.

“Historically, police have tracked your technology – your car, your phone, your computer,” he says. “You can turn off your phone, you can leave your car at home; you can’t turn off your face.”

Story by David Fishman and video by Michelle Kim, both students at Northwestern University.