Why was croatoan scratched on a tree?

The Lost Colony of Roanoke, in Virginia, was first the English settlement in the New World. After not receiving supplies from England for two years, they sent their leader back to England to bring them supplies. He returned two years later to find the colony had vanished. Not a single person was there and there was no evidence of a fight. The only evidence they found was the world "CROATOAN" carved on a tree, and "CRO" on another tree nearby.