The US National Security Agency (NSA) is building a database of photos drawn from online sources, such as Facebook, as well as private communications tapped by the NSA, as part of a facial recognition project.
The project is capturing millions of images every day, including an estimated 55,000 of "facial recognition quality".
The programme is ostensibly aimed at identifying and tracking down terrorist targets, but the sheer volume of images involved indicate that its true aims are far broader.
The revelation is just the latest to arise from documents leaked by NSA whistleblower Edward Snowden.
One presentation slide explaining the programme claims: "It's not just the traditional communications we're after: It's taking a full-arsenal approach that digitally exploits the clues a target leaves behind in their regular activities on the net to compile biographic and biometric information [in order to] implement precision targeting."
A NSA spokesperson told the New York Times that the organisation was forbidden from tapping official image databases, such as the databases of state driving licence agencies and the US passport office - although one of its partners, such as GCHQ, would not be similarly constrained.
According to the New York Times, the presentations indicate that the NSA is able to match two photos of the same person, if the person is bearded in one image and clean-shaven in the other. Other image recognition technology is used to analyse the backgrounds in order to try and identify their locations.