Why is Jefferson in jail awaiting execution in the novel, A Lesson Before Dying?

On his way to a bar one October day, Jefferson accepts a ride from two other young black men, Bear and Brother. Bear and Brother decide to stop to buy liquor but have no money between them. The two think the store owner, Mr. Grope, will allow them to get the liquor on credit. When they ask him, he disagrees, and they begin to argue. Already drunk, Bear starts around the counter. Mr. Grope gets his gun and begins to shoot. Before Jefferson knows what has happened, all three men are dead. Confused, he is still in the store when two white men find him. He gets blamed for robbing and killing the storeowner.