Rob Evans, a Guardian journalist, submitted a freedom of information request to see the letters but this was denied by the information commissioner.

BBC: Prince Charles letters to government must be published

The FDA replied to a request for comment with three letters: "FOI, " referring to the Freedom of Information Act.

FORBES: The FDA's Conflict Of Interest Problem

The IRS did not respond to CNN's request for comment regarding the letters.

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When I review institutional portfolios, I regularly request, receive and review non-public SEC deficiency letters of firms handling client assets.

FORBES: SEC Conceals Far More Crime Than You Ever Imagined

The USA PATRIOT Act expanded the purpose of National Security Letters (NSL) to allow government agencies to use them to request any data relevant to an investigation of terrorism or an intelligence activity without court oversight.

FORBES: The Sheep Stop Here: Another Church Committee or Full Review of Privacy Laws Needed?

Going a bit further, the Comptroller's office released two previously undisclosed letters from 1993, the first acceding to a bank's request to hold a condominium for its own use and the second allowing the bank to rent it out when it was not needed.

ECONOMIST: American banks and property