Is a FHA loan the same as a tax-exempt mortgage revenue bond?

I received the 2008 First Time Homebuyer Credit (which is more like an interest free loan repayable over 15 years). My loan is a FHA loan through CountryWide and now through US Bank (it was transfered). I was under the impression that an FHA loan is not the same as a tax-exempt mortgage revenue bond. However, I just received a letter from the IRS today indicating I was not eligible for the credit. I called the IRS and waited on the phone for over an hour because I wanted to find out why I wasnt eligible- however, I was only told they don’t have that information and that I need to repay the "interest free loan" all back immediately.

I meet all the other criteria to receive the loan so I am thinking they are considering a FHA loan a rtax-exempt mortgage revenue bond. Here is the criteria for the credit:

I earn under $35,000 a year.

I did not buy the home from a close relative.

It is still my main home .

I am a citizen of the US.

I do not live in the District of Columbia and did not take the District of Columbia credit.

I do not own or have never owned another home.

So the only thing left is:

Your home financing comes from tax-exempt mortgage revenue bonds.

Could someone please help? Thanks!