Book of Enoch
According to "The Expositor's Bible Commentary", Azazel is the Hebrew word for scapegoat. This is the only place that the Hebrew word is found in the whole Hebrew Old Testament. It says that the Book of Enoch, (extra-biblical Jewish theological literature, dated around 200 B.C.) is full of demonology and reference to fallen angels. The EBC (Vol 2) says that this text uses late Aramaic forms for these names which indicates that The Book of Enoch most likely relies upon the Hebrew Leviticus text rather than the Leviticus text being reliant upon the Book of Enoch.