Yahoo reportedly aided the US government in scanning the email accounts of hundreds of millions of its Yahoo Mail users.

According to Reuters, Yahoo secretly built a custom software program to search all of its customers incoming emails for specific information provided by US intelligence officials. This was in response to a classified edict from the NSA or FBI received by Yahoos legal team.

What exactly the information being searched for was – and what, if any, data was handed over – remains unknown, but its a possibly unprecedented example of online surveillance. Google and Microsoft told Reuters that they had never carried out similar searches on their own email services, while Yahoo said in a statement: “Yahoo is a law abiding company, and complies with the laws of the United States”.

It makes for an interesting comparison with the public struggle between the US government and Apple earlier this year, after Apple refused to create an encryption ‘backdoor’ for accessing data stored on any iPhone.

Just last month, Yahoo revealed that information on at least 500m users had been stolen back in 2014. It blamed a “state-sponsored actor” – from an unspecified country – for the data breach, though it appears to have been an entirely separate incident.

Yahoo is currently in the process of being acquired by US telco giant Verizon.