You need to back up your claim. You have to prove that your life will be in danger if you are sent back to your country. Threatening letters helps, if you can get newspaper clipping, that helps also. If the US Dept. of State human rights report of your country supports your claim, that is even better. At the end of the day, the IO has to be satisfied that your life could be in danger if you are sent back to your country. If the IO is satisfied, you will get asylum.