Do you have the proof that the letter was sent? That is, there must be 'proof of service' in some way, and the proof of service would include whether or not the actual subpoena went along with the letter.
I realize that if they sent the letter without the subpoena, that it would look very bad for them, except if everyone they sent the letter to was involved in a cover-up.
So, just curious, is there any proof of service?
