Thread by @StilichoReads

Did the Roman Empire have a secret police/intelligence service? Like many states, it had several, with duties that overlapped and varied over time. These included the Speculatores, Frumentarii, Areani, Agentes in Rebus, and even the Corps of Notories.
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It should of course be rembered that much of this information is tentative - such units are, by definition, secretive. Still, information comes through in small amounts in sources and on the gravestones of soldiers.

The Speculatores were military scouts. Although mostly responsible for traditional reconnaissance, at least the Praetorian Guard's Speculatores under Galba, Otho, and the Flavian Dynasty, often worked in plainclothes and were responsible for close protection and assassination.

Julius, the merciful centurion who escorted St Paul to Rome, is described in Acts 27:1 as being of the "Augustan band." There was a Legio II Augusta, but as it was stationed far from Palestine it is possible this man was one of these Imperial bodyguards on secret police duty.

The Frumentarii were originally soldiers responsible for the collection of grain - their name literally comes from Frumentum, grain - and this job often involved interacting with civilians in small groups or alone, at great distance form the main army.

Although they always served as supply sergeants and messangers, under the Emperor Hadrian they began to serve as secret police. Based at the Castra Peregrina in Rome, there were Frumentarii in every province. A story shows just how deep their ability to gather intelligence went:

The wife of an imperial official wrote to him, claiming that he was ignoring her and spending too much time at the bathhouse. The Frumentarii secretly read the letter, and Hadrian reproached him for it. The startled official wondered if his wife had wrote the emperor as well!

This was not a secret assignment, at least after retirement, and Frumentarii were sufficiently proud of their service to note it on their gravestones and to make inscriptions honoring the genus, or guardian spirit, of the Castra Peregrina.

We know far less about units responsible for foreign intelligence, but they certainly existed - Ammianus Marcellinus, writing in the 4th century, describes the Areani, a group responsible for collecting information north of Hadrian's wall.

Unfortunately for Rome, these men were bribed and went native, providing information to local chieftains on the Roman army. It is almost certain that similar units existed along the more important frontiers of the Rhine, Danube, and border with Persia.

Areani may translate to "of the sheepfolds" but there is another possibility. A tablet from the Roman fort at Vindolanda, on Hadrian's Wall, says "Miles Arcanu[s]," or secret soldier. It is possible this was the unit's true name, miscopied by a medieval scribe.

The Emperor Diocletian disbanded the unpopular Frumentarii, replacing them with the civilian Agentes in Rebus, or general agents, along with the Corps of Notories. However, as Russians say about the KGB and FSB - "new name, same friendly service."

Like the Frumentarii, the Agentes were not only a security service, working also as couriers, central bureaucrats, and even ambassadors. Along with the rest of the Late Roman civil service, they were organized along military lines.

The notories had, at least in some cases, similar tasks. One senior notary under Constantius II, Paulus Catena - "Paul the Chain" - earned his nickname from his favorite method of torture on the emperor's suspected enemies. He was feared throughout the empire.

Many of those he had killed were in fact loyal, honorable men. However, like many totalitarian thugs throughout history, he ended up hoist by his own petard, and was himself tried and burned alive.

Both agencies continued in the Eastern Empire, and the Agentes even remained in the government of Ostrogothic Italy. This is only a survey of these organizations, but it sheds a light on the omnipresent grimmer side of empire.