An agnostic is not necessarily an atheist, but he can be. I, for example, am an agnostic atheist. Agnosticism addresses a question of KNOWLEDGE and atheism addresses a question of BELIEF; an agnostic atheist does not KNOW whether there is a God, but he does not BELIEVE that he does. These two terms (agnostic and atheism) are NOT mutually exclusive, they actually often go hand in hand (since I do not know how one could KNOW that there is no God).
And by the way: ATHEISM IS NOT THE BELIEF THAT THERE IS NO GOD. IT IS SIMPLY THE REJECTION OF THE CLAIM "GOD EXISTS". Not believing something is NOT the same thing as claiming it is false.