A Houston detective says he couldn't see the child pornography that was stored in a registered sex offender's Gmail account, but Google could, and now the accused now faces justice because Google searched out and shared the obscene content with law enforcement agencies.

Privacy advocates wonder now, though, just how far the search engine company's probing will go in the future.

John Henry Skillern, 41, was arrested and charged with the possession of child pornography after Houston investigators followed up on a tip from Google and executed a search warrant at the registered sex offender's property.

More than 500 million users rely on Google's Gmail to privately and securely exchange media. That massive repository of data is where Google found the content that lead to Skillern's arrest. No one seems to object when emails are scanned for malicious software, but the issue turns gray and contentious from there, Robert Gellman, a privacy and information policy consultant, said to Mashable.

"Reading email to turn over to law enforcement opens a whole different can of worms," Gellman said. "No one defends child porn, but the principle that an email provider will read mail looking for criminal activities is problematic. It raises concerns over what standards apply and which crimes are included."

Addressing privacy concerns, Google said the fingerprinting technology it uses to identify imagery that depicts the abuse of children isn't used to search out any other criminal activity. Each offending image is tagged and identified by Google servers, without human scrutiny involved on Google's end. Google said it has been using the technology since 2008.

Google used a burglary plot as an example of the type of criminal activity its fingerprinting technology doesn't search out, but privacy advocates have wondered how long it'll be before the company expands its net to catch other species of criminals.

Mark Sigal, managing director of BrightStreet Ventures, described Google's probes of Gmail as "a Faustian bargain between surveillance and security." Whether Google agrees to it or not, it may find itself increasingly turning over all sorts of user data to law enforcement agencies -- a federal judge recently authorized a warrant to force Google to hand over Gmail data from suspects that were implicated in a money laundering scheme.

It's a slippery slope, but it may not be fallacious. The norm of Google searching out child predators could move to pinpointing violent plots, to tax fraud, to eventually being leveraged to target political activists.