Yahoo Inc last year secretly built a custom software program to search all of its customers' incoming emails for specific information provided by U.S. intelligence officials, according to people familiar with the matter.

The company complied with a classified U.S. government demand, scanning hundreds of millions of Yahoo Mail accounts at the behest of the National Security Agency or FBI, said three former employees and a fourth person apprised of the events.

Some surveillance experts said this represents the first case to surface of a U.S. Internet company agreeing to an intelligence agency's request by searching all arriving messages, as opposed to examining stored messages or scanning a small number of accounts in real time.

It is not known what information intelligence officials were looking for, only that they wanted Yahoo to search for a set of characters. That could mean a phrase in an email or an attachment, said the sources, who did not want to be identified.

Reuters was unable to determine what data Yahoo may have handed over, if any, and if intelligence officials had approached other email providers besides Yahoo with this kind of request.