Frumentarii

Frumentarii were officials of the Roman Empire, originally wheat collectors, who also acted as the secret service of the Roman Empire in the 2nd and 3rd centuries.

There are two main sources of information about the frumentarii, inscriptions on gravestones and anecdotes where the actions of individual frumentarii are mentioned by historians. From what is known of the Frumentarii, they always worked in uniform. The Empire was based on patronage, not on ideology (until Theodosius I). From inscriptions; one of the few things known about the frumentarii is that they were mostly attached to individual legions, except for a few centurion frumentarii. Attachment to individual legions suggests that their main function was, as the name suggests, to service those legions with supplies.

Frumentarii appear to have spent a lot of time travelling and had a base in Rome at the Castra Peregrina. Frumentarii were obviously proud of their status if they put the rank on their gravestones. There are a number of inscriptions honouring the genie of the Castra Peregrina, this suggests that the frumentarii had high moral and social status.

It had been long-standing policy of the Roman legions and armies of occupation to utilize informers and spies, but never in an organized fashion. This was especially true in the city of Rome which was rife with whispers and endless conspiracies. There are two inscriptions of "frumentario canaliculario" found at Arles and Córdoba which suggest that some frumentarii had special knowledge of inland navigation. For all armies, the most important unit of military intelligence was an enemy's geographical location. This included mapped land and communication routes, enemy legion sizes, landmarks, and strategic objectives such as granaries or farms. The Antonine Itinerary might be the product of the frumentarii. Titus used special messengers and assassins of the Praetorian Guard to carry out executions and liquidations (the Speculatores); however, they belonged to the Guard and were limited in scope and power.

...
Wikipedia