[anigbrowl]

Does the gov't need a second warrant to open a letter-in-a-letter?

Interesting question; I'm guessing no, unless the interior letter contained obviously privileged material (eg 'Dear Father, I hope you are well; enclosed is a letter I received from your lawyer after you departed, which I forward to you unopened. Your Loving Son.'), in which case a warrant would probably not issue for the contents.

[fnordfnordfnord]

http://www.nytimes.com/interactive/2013/06/30/us/30postal-ma...

Section I B - allows the contents of unsealed classes of mail "as allowed by law"

Section II B - "...Mail Covers do not authorize the search, seizure, or opening of any class of mail."

Section III B 6 - addresses attorney-client privilege.

[anigbrowl]

None of which is responsive to the question asked above, which was about a letter that was lawfully opened pursuant to a warrant, but which contained a second, sealed letter.