SPQR by John Maddox Roberts
Murder isn't unusual in the district of Decius Caecilius Metellus the younger. As the equivalent of a policeman in the ancient Rome, he's used to many deaths and violence. But no one knows what happened. Not even the people who own most of the criminals. When more powerful and rich people die before their time, Decius gets suspicious: Everything leads to the two consuls and the guest in Rome, the Persian Prince... But a conspiracy is as unhealthy for the victim as for the one who tries to reveal it.
Other parts: The Catiline Conspiracy, Sacrilege, The Temple of the Muses, Saturnalia, Nobody loves a Centurion, The Tribune's Curse, The River God's Vengeance, The Princess and the Pirates, A Point of Law, Under Vesuvius, Oracle of the Dead, The Year of Confusion,