> In Boston it had nothing to do with slaves because Massachusetts was not a slave state when they created a police system in the 1830s.

https://en.wikipedia.org/wiki/Fugitive_Slave_Act_of_1793

https://en.wikipedia.org/wiki/Fugitive_Slave_Act_of_1850

"It required that all escaped slaves, upon capture, be returned to the slave-owner and that officials and citizens of free states had to cooperate."

Boston's police department was founded in 1854.