Final answer:
The Praetorian Guard was involved in political repressions during Tiberius' rule, and although specific texts don't directly mention who imprisoned Agrippina, it's implied that the Guard played a role in such actions.
Step-by-step explanation:
The Praetorian Guard was an elite unit of the Imperial Roman army whose members served as personal bodyguards and intelligence for Roman emperors. During the reign of Emperor Tiberius, the Praetorian Guard had significant influence and was involved in various political and physical suppressions of the emperor's perceived enemies. Agrippina the Elder, wife of Germanicus and mother of the future Emperor Caligula, was one of the notable figures who was imprisoned on orders that could be attributed to the machinations within Tiberius' regime, where the Praetorian Guard played a role.
The historical accounts from LiberTexts also describe the chaotic state of the Roman entertainment scene, which resulted in laws enacted to regulate actors and public disturbances, leading Tiberius to ultimately banish certain faction leaders from Rome. The political conflict between Germanicus and Piso, as well as Tiberius's dark rule, has been well documented by historians such as Tacitus and Suetonius. The Praetorian Guard, under such circumstances, was seen as a tool of repression used by Tiberius to quell dissent and maintain his rule.
Regarding the specific question about the Praetorian Guard and Agrippina, the texts do not provide a straightforward answer about who exactly put Agrippina in prison. However, given the context of Tiberius' rule and the significant role played by the Praetorian Guard, it is likely that they played some role in her imprisonment, as well as in the suppression of other enemies of the emperor.