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http://history.howstuffworks.com/american-civil-war/underground-railroad2.htm

In the Fugitive Slave Act of 1793, the federal government gave local authorities in both slave and free states the power to issue warrants to "remove" any black they thought to be an escaped slave. It also made it a federal crime to help a runaway slave. The act was rarely enforced in non-slave states, but in 1850 it was strengthened with higher fines and harsher punishments. On top of that, slave hunters could legally claim that any black person they saw was an escaped slave, which not only terrorized free blacks but outraged many white people. Northerners were horrified by rumors of slave hunters luring preschool-age free black children onto boats and shipping them to the Deep South.