Then God asked Cain where's his brother. Cain said he didn't know, was he is brothers keeper?
So God asked him what he had done and that his brother's blood cries out to Him from the ground. And then told Cain that he was under a curse.
God told Cain that when he would work the ground that it would no long yield its crops for him and that he would be a restless wanderer on the earth.