Who caused Rosaleen's head injury and why?

When Rosaleen went to town in order to register to vote, she receives repeated derrogatory remarks from three white men on the street. Rosaleen's response is to pout tobacco spit all over their shoes, and act for which she receives a beating. None-the-less, it isn't until she is taken to the police department that her head is injured. The men have followed the police car to the station, and when she is removed from the car in handcuffs, the men proceed to beat her again. Her head is severely injured during this assault.