What circumstantial evidence suggests that they were guilty of this crime?

Brown and Hamilton tell Solomon that they are members of a circus and are interested in his violin skills. They tell him that he will be paid one dollar per day, and three dollars for every performance if he accompanies them to New York City and Washington DC. One night at dinner, they drug Solomon and give him to the slave dealer James H. Burch. Solomon spends much time wondering about their complicity in the whole matter: they were very kind to him, but he ultimately decides that they must have been in on it.