The Alienist is a crime novel by Caleb Carr first published in 1994 and is the first book in the Kreizler series. It takes place in New York City in 1896, and includes appearances by many famous figures of New York society in that era, including Theodore Roosevelt and J. P. Morgan. The sequel to the novel is The Angel of Darkness. The story follows Roosevelt, then New York City police commissioner, and Dr. Laszlo Kreizler, as their investigative team attempts to solve gruesome murders through new methods including fingerprinting and psychology. The first murder victim investigated is a 13-year-old immigrant who has had his eyes removed, his genitals removed and stuffed in his mouth, and other injuries. The investigators deal with various interest groups that wish to maintain the status quo regarding the poor immigrant population in New York City.

Paul Kelly was an American mobster and former boxer, who founded the Five Points Gang in New York City. He had started some brothels with prize money earned in boxing. Five Points Gang was one of the first dominant street gangs in New York history. Kelly recruited young, poor men from the ethnically diverse immigrant neighborhoods of Lower Manhattan. The Five Points Gang included some who later became prominent criminals in their own right, including Johnny Torrio, Al Capone, Lucky Luciano, Meyer Lansky and Frankie Yale.