It's a very common misconception, but yes you can be both an atheist and an agnostic. The commonly held error is that you have theism and atheism on opposite ends of a spectrum and agnosticism is a kind of middle road between them. This isn't really the case. Theism/atheism and gnosticism/agnosticism are addressing different issues, the first belief and the second what you know or claim to know. I am an agnostic atheist because I do not hold a belief in God or gods, but I do not claim to know if they exist or don't exist.
I posted this graphic in another thread but will post it here again. Hope it clarifies things.
Oh okay, I thought an agnostic thinks it impossible to know the truth in matters such as God and the future life with which Christianity and other religions are concerned. Or, if not impossible, at least impossible at the present time. And a atheist is one who is convinced that God does not exist.