Early in the nineteenth century, Philadelphia physician Jesse Torrey compiled a broad range of interviews, narratives and his own first-hand observations into an illustrated volume appropriately entitled, A Portraiture of Domestic Slavery in the United States, published in Philadelphia in 1817. Torrey's book contained several engravings related to slave kidnappings, a profitable underground industry that thrived during the period. Free blacks were especially vulnerable to kidnappers, who would sell them into slavery using forged documents.