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.