Yahoo's tool to scan the contents of customer emails came as a result of a U.S. Foreign Intelligence Surveillance Court order last year, and was created by adapting otherwise benign software, a report indicated on Thursday.
Government officials were hunting down a "signature" linked to the communications of a state-sponsored terrorist organization, sources told the New York Times. To meet demands, Yahoo adapted a system normally meant to catch spam, malware, and child pornography, enabling it to find messages with the signature for sharing with the FBI.
That collection is no longer ongoing, two of the sources said. Scanning traffic was allegedly necessary because while the terrorists were thought to be using a unique identifier, investigators didn't know which email accounts were associated.
Yahoo has denied the existence of the scanning tool, claiming that the technology described in a Reuters article "does not exist on our systems."
An Apple spokesperson told BuzzFeed News that it has "never received a request of this type," and that even if it had, it would fight it in court. A Microsoft representative insisted the company has never performed a similar scan, while a Google spokesperson said the firm hasn't received this kind of request and would refuse.
The truth of the matter may be difficult to discern. Requests like the one Yahoo reportedly received often come with a gag order, preventing people from disclosing that a search took place. Yahoo and other companies like Apple and Facebook have denied providing "direct access" to their servers for the National Security Agency's PRISM mass surveillance program, or even being aware of PRISM, but were known to have had their data harvested -- willingly or otherwise.