Google is monitoring our email activity for illegal images, it has been revealed, after a convicted sex offender was arrested following the firm locating images of child abuse in his Gmail account.
During an automated search, Google found the illegal images and reported them to the US non-profit National Centre for Missing and Exploited Children.
Detective David Nettles of the Houston Metro Internet Crimes Against Children Taskforce praised the firm for the role it played in the arrest.
"He was keeping it inside of his email. I can't see that information, I can't see that photo, but Google can. He was trying to get around getting caught, he was trying to keep it inside his email. I really don't know how they do their job. But I'm just glad they do it," he said.
Google refused to reveal the exact details of the searches it carries out, but it is thought that the automated image search works by comparing "hashes" of images rather than the files themselves.
Each hash is a unique code created by running an image through an algorithm. The hash is then compared against a database of hashes for known child abuse pictures. A match means that an image is likely to contain illegal content and ensures that Google does not need to compile a database of illegal images itself.
Google has previously suggested that it carries out searches on content, but did not reveal if this was public or private data. Its chief legal officer, David Drummond, revealed in an interview with The Telegraph last year that the firm was tackling illegal images.
"Since much of this illegal material is circulated repeatedly - making the crime infinitely worse for the victims - we have built technology that trawls other platforms for known images of child sex abuse," he said.
While the latest news may make some users uneasy at the thought of their privacy being compromised, few would deny the importance of restricting the circulation of child abuse images online.