Detective Alistair 'AJ' Castlereagh was born in Harrisburg, the capital city of the Commonwealth of Pennsylvania. His father, Stuart Castlereagh, was a moderately successful investment banker who would become a State Senator in the Pennsylvania General Assembly when AJ was thirteen.

Though not fantastically rich, the wealth Stuart Castlereagh gained through his business ventures and his position as a State Senator allowed AJ to have a modest head start in several areas. He attended public school but received excellent private tutoring, and later was accepted into the University of Pennsylvania with a high SAT score and all tuition once again paid for by his father.

However AJ and Stuart did not see eye to eye on many occasions. The divorce of his parents in 1996 strained relations between AJ and his father and Stuart was not pleased when AJ began to study psychology rather than business, as Stuart had hoped. Nevertheless, they did continue with a healthy if sometimes tense relationship.

Two major events would shake their relationship to its foundation, however. First, AJ expressed his ambitions to become an FBI special agent as his older sister, Cassandra, had done previous. Cassandra's choice had alienated her from their father and a similar cycle began with AJ. Then in late 2002, Stuart was implicated in a major government scandal involving improper awarding of a waste management contract to a banking firm that later turned out to be a front for money laundering. While Stuart Castelregh was never charged, both his political career and his son's faith in him were left in shambles.

Disgusted by this turn of events, and the secret revelation that his sister and role model Cassandra had already suspected their father of wrongdoing but did nothing, AJ entertained the idea of enlisting with the Sierra Falls PD. This would allow him both to pursue his ambition of being a law enforcement officer and be close to his mother, now residing in Santa Maria County.

In 2006 Castlereagh wrote the detective exam for the CIDB, passing with above average marks. Satisfied with his work as a patrol officer, he felt that he could put himself to better use solving serious crimes and actively pursuing criminals.

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