Agnostics are pompous asses who want to pretend like they are somehow the reasonable middle ground. First of all, you are either a carpenter or not a carpenter, if you are not a theist, then you are an atheist. Agnostics and atheists believe the exact same shit, namely, they are not theists based on what they know now. Agnostics just lie the burden of proof on the wrong end. If a god came cruising down from the sky on a golden cloud for all to see, I am pretty sure virtually all atheists would believe in him, save a few loonies of course. Does that make them agnostic now?
Actual certainty in any held belief is a pretty high bar, and it is pretty stupid to form a separate category on the lack thereof.
It isn't the same thing though, Agnosticism is a part of Atheism, but a non-Agnostic Atheist would say "There can't be any Gods, that makes no sense" while an Agnostic Atheist would say "How the fuck would I know?"
Are all Agnostics also Atheists? -Yes, because they don't believe in a certain God.
All are Atheists also Agnostic? -No, they believe God concept is completely retarded.
It is a common misconception tho, you are either a Theist or Atheist, there is no middle ground.