Many of us have far, far too much experience with FAA abuse and too many years dealing with the FAA to buy into the bull**** of kinder and gentler compliance based philosophy. Anything you say can and absolutely will be used against you, and no, the FAA is NOT interested in your explanation of innocence unless it yields ammunition to use against you.
Don't call the tower. Don't respond to that letter of investigation, in the hopes the FAA will bless you through teary eyes and send you on your way. Call an attorney. You'll hear all day long that you don't need one. The person who tells you that does not have your best interests at heart. Know it. Embrace it.
The FAA is not your friend.