What was the Burning Bush?

When Moshe saw the bush ablaze yet not being eaten, he came to see what it was all about. The pasuk says there was an angel in that bush. Then Hashem called out to him “Moshe , Moshe” and Moshe answered “Here I am”. Then he was told to take off his shoes, and then, only then, Moshe hid his face so as not to gaze upon Hashem. We discussed whether it was an angel speaking throughout, representing Hashem, or did it begin with an angel but the actual conversation was with Hashem himself, or perhaps even farther – that Hashem himself came down and was burning in the bush. The pasuk in V’zos HaBracha calls Hashem “the Dweller in the s’neh” which seems to indicate that Hashem Himself was the fire. So why was there an angel at first?