If you look up the website of the particular US embassy that you applied to, almost certainly on that website, there will be a 'Contact us' section. In your place, I think I would send the same message by all the written means they offer, e.g. email, ordinary letter, and fax (if it's there), stating that you are in urgent need of this information and therefore are sending duplicates, and that you would appreciate a reply by x date. Keep dated copies of everything you send. Then you can show INZ that you have done everything you possibly could.