When asked if agnostics are atheists, Bertrand Russell answered,
“No. An atheist, like a Christian, holds that we can know whether or not there is a God. The Christian holds that we can know there is a God, the atheist that we can know there is not. The agnostic suspends judgment, saying that there are not sufficient grounds either for affirmation or for denial.”1
He does concede that certain agnostics who think the existence of God so improbable as to not be worth considering are not far removed from atheism.
At any rate, I recently discovered that not every atheist allows for the category of agnostic.
1 Interview in Look magazine, November 3, 1953