From Wikipedia: Slave patrols

 One of the origins of policing in the United States.

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Slave patrols first began in South Carolina in 1704 and spread throughout the thirteen colonies, lasting well beyond the American Revolution. As the population of enslaved black people boomed, especially with the invention of the cotton gin, so did the fear of resistance and uprisings by the enslaved. The development of slave patrols began when other means of slave control failed to quell enslaved people's resistance. Their biggest concern were the enslaved being held against their will on the plantations since that is where enslaved populations were highest. Initially, incentives were offered to whites such as tobacco and money to urge whites to be more vigilant in the capture of runaway enslaved persons that had escaped. When this approach failed, slave patrols were formally established. 

Laws were put into place to regulate the activities of both blacks and whites. Black persons were subjected to questioning, searches, and other harassment. Slaves who were encountered without passes from their white "master" were expected to be returned to their owners, as stated in the slave code. Punishment for runaway slaves, such as whippings and beatings, could be expected. More than floggings and beatings, however, enslaved people feared the threat of being placed on the auction block and being separated from their families. If caught by patrols and returned to their masters, being placed on the auction block was an option for masters who no longer wanted to deal with their "non-compliant" slaves. During these times, slaves were often neglected and mistreated despite having permission to travel.