The release stems from a loosening of restrictions by the federal government on publicly sharing National Security Letters. The requests made by the FBI were made to Google to identify 21 customer accounts and related account data. In 2015 Congress passed the USA Freedom Act, which removed an indefinite gag restriction on the letters and allowed companies such as Google to disclose some information about the requests they receive. Google, Yahoo, Microsoft, and others have challenged the restrictions on the disclosure of NSLs consistently, seeking the ability to be more specific about the number of letters they receive.
Source: https://threatpost.com/google-discloses-contents-of-eight-national-security-letters/122488/