Decius starts by consulting the fortune tellers and priests of foreign cults most prominent among his aristocratic wife's friends (these are regularly expelled from the city, but a few always bribe the aediles into letting them stay).
One he consults, a Greek scholar named Ariston, admits that he schooled Ateius Capito in ancient religious practices and certain magic rituals, but denies giving him any dangerous knowledge.
Though Capito was reviled by the city for performing the curse, a tribune of the plebs is supposed to be sacrosanct from violence, and the populace is outraged almost to the point of rioting.
While investigating his past career, Decius is struck by the fact that, while in office, Capito devoted himself wholeheartedly to opposing Crassus' war with Parthia, to the exclusion of all other business.
Julia points out that a tribunate is meant to be the start of a political career, yet Capito committed professional suicide by performing the curse, which means (assuming he wasn't insane) he must have had some alternate plan for the rest of his life.
The mob is further outraged when news arrives from Egypt that King Ptolemy Auletes has put his daughter, Berenice, to death, for rebelling against his rule, along with several thousand of her followers, including several prominent Alexandrian citizens with friends and connections in Rome.
After a second, more careful review of Capito's records, Decius solves the case, and a short time later, he, Milo, and Pompey storm into the Egyptian embassy, to confront the ambassador, Lisas.
After performing the curse, Capito faked his death by having a young slave dressed in his clothes and mauled by the Nile crocodiles kept as pets inside the Embassy, where he has been hiding out ever since, waiting to make a dash to his new retirement villa in Egypt.