[_Nat_]

It might have been an application of [18 U.S.C. §2705-a-1-B](https://www.law.cornell.edu/uscode/text/18/2705):

> where an administrative subpoena authorized by a Federal or State statute or a Federal or State grand jury subpoena is obtained, delay the notification required under section 2703(b) of this title for a period not to exceed ninety days upon the execution of a written certification of a supervisory official that there is reason to believe that notification of the existence of the subpoena may have an adverse result described in paragraph (2) of this subsection.

This would seem to sync up with what the story said:

> With help from Ken, we soon received a "preservation letter" demanding that we preserve for a period of 90 days "any and all records and other evidence, [...] In addition, the letter effectively gagged us, saying that we were not to disclose the existence of the letter "in any manner that could alert the user" of the account.