The FBI has spent years quietly building a huge trove of eyeball data

“Eyes,” as the old adage states, “are the windows to the soul.” For the Federal Bureau of Investigation, however, eyes are the next big thing in biometric data tracking.

The FBI has quietly been expanding a 2013 pilot program which has reportedly collected scans of irises belonging to over 400,000 arrestees to date. The program is part of of the FBI’s “Next Generation Identification” initiative. While the bureau has made no secret of its interest in biometrics, the fact that it has already amassed a collection of hundreds of thousands of unique iris scans was only recently made public, thanks to an exhaustive report by The Verge.

For law enforcement, eye-scanning offers some very real benefits over fingerprinting, while operating under the same basic premise: An iris—the colored circle surrounding the eye’s pupil—contains patterns of lines and fluctuations unique to an individual, much like their fingerprint. Scanning a person’s eye, however, can be done quickly, and with no touching. In other words, while irises may not currently be all that useful in crime scene forensics (people aren’t leaving their eyeball prints on the grip of a pistol) they’re perfect for quickly adding to, and identifying members of, a database—no ink stained fingertips necessary.

As The Verge points out, the ability to scan and identify a person’s unique iris signature is nothing new—it’s already been used by the military, and private security companies. Instead, it’s the FBI’s efforts to pool iris scans into a central database that’s helping to normalize the technology for widespread domestic law enforcement. But the bureau isn’t doing this alone: They’re reportedly working in cooperation with agencies in Texas, Missouri, and California, all of whom are able to access and add to the collective pool of iris scans.

That growth has alarmed some rights experts. They worry that what began in 2013 as a limited FBI initiative has since ballooned into something else. Something that, to-date has reportedly been spared the sort of outside regulation and reporting ordinarily associated with larger data-collection initiatives, by virtue of its status as a pilot program.

As iris identifying technology continues to evolve, concerns are likely to grow as well. Already researchers at Carnegie Melon University have developed an iris scanner that can achieve results at 40 feet away. It’s an advancement that alarmed at least one research participant, who explained, “I feel negatively about a remote iris scan because I want there to be some kind of interaction between me and this system that’s going to be monitoring me.”

While there’s no indication that the FBI is engaging in this level of scanning, it’s easy to understand why the aggregation of scan data is alarming to many—especially given it the apparently minimal external oversight placed on FBI’s program. The removal of a physical interaction from the process of data collection only plays to dystopian paranoia, in which everyone risks being subject to involuntary iris identification.

While that doesn’t appear to be the case now, the newly revealed existence of the iris database certainly doesn’t assuage those fears, either.