Google sent a tip-off to child protection officials after spotting images of child abuse on the Gmail account of a convicted sex offender, who was then arrested by police in Houston, Texas.
"I can't see that information, I can't see that photo, but Google can," investigator Detective David Nettles said.
Gmail is Google's own email service and has over 400 million users worldwide. The search engine giant updated Gmail's terms and conditions earlier in the year to acknowledge that email accounts were scanned for content to provide "personally relevant" adverts to users.
The company has also spoken of being "proactive" in its work to remove images of child abuse from its search results, working with the National Centre for Missing and Exploited Children in the US to report the existence of images. The company also helps fund the Internet Watch Foundation, which is dedicated to finding and removing child abuse images from the web.
However the arrest has prompted questions about privacy on the web and the reach of Google. Emma Carr, acting director of privacy group Big Brother Watch, said: "When a company takes a commercial decision to scan the content of messages, it is the next logical step that governments and law enforcement will want to take advantage of that.
"With the rate that Gmail messages are scanned, and the fact that all US companies are bound by US law to report suspected child abuse, it is hardly surprising that this individual has found themselves on the wrong side of the law.
"However, Gmail users will certainly be interested to know what action Google proactively takes to monitor and analyse Gmail messages for illegal content, including details of what sorts of illegal activity may be targeted. Google must also make themselves very clear about what procedures and safeguards are in place to ensure that people are not wrongly criminalised, for instance, when potentially illegal content is shared but has been done so legitimately in the context of reporting or research."
A Google spokesperson said “Sadly all Internet companies have to deal with child sexual abuse. It’s why Google actively removes illegal imagery from our services -- including search and Gmail -- and immediately reports any abuse to the National Center for Missing and Exploited Children, which collaborates with law enforcement.
"This evidence is regularly used to convict criminals. Each child sexual abuse image is given a unique digital fingerprint which enables our systems to identify those pictures, including in Gmail. It is important to remember that we use this technology to identify child sexual abuse imagery, not email content that could be associated with general criminal activity (for example using email to plot a burglary).
"We can’t comment on the specifics of an ongoing criminal investigation.”