Agnostic atheists lack belief in the existence of a deity insofar as their beliefs are determined by rational investigation of empirical evidence.

Strict agnostics are not even comfortable going this far. Empirical inquiry is our sole epistemic endeavor, and this is expanding the definition of empirical inquiry to encapsulate axioms that are not derived from observation and experimentation (this is a controversial position that orients towards scientism.)

That said, I can be called a fundamentalist of empiricism. I don't believe the fruits of empirical investigation have been compelling enough to justifiably lack belief in the existence of a God, even if I consider the existence of God as improbable as the existence of the Tooth Fairy. In this sense I disagree with Dawkins's argument that the distinction between agnosticism and atheism is one of probability allotment. Like Dawkins, I am 99.9999_% certain God does not exist, but unlike him I do not consider this compelling evidence to justifiably withhold belief.

Note that for all intents and purposes I am practically indistinguishable from most agnostic atheists. I only differ in that I have a more militant approach to empirically-derived beliefs.