Yahoo built custom software to comply with a government request to monitor all of its users' emails, according to former employees.
That meant hundreds of millions of Yahoo Mail accounts were scanned, either for the NSA or FBI, the former employees told Reuters.
This is the first known time a U.S. company agreed to a spy agency's request to search all emails, instead of storing them or scanning a select number of accounts.
Why this one stands out
U.S. phone and Internet companies have turned over bulk metadata to authorities before (as Edward Snowden revealed). But Yahoo's alleged surveillance stands out for its scope, and that the company built software to do the job itself.
Yahoo CEO Melissa Mayer reportedly complied with the directive because the company thought it would lose the inevitable legal battle, despite legal experts telling Reuters that Yahoo could have challenged it.