Early in a.d. 29 died Iulia Augusta, the mother of Tiberius, at the age of eighty-six. That she had been a protectress of the house of Germanicus appears from the fact that, almost immediately after her death, letters were despatched to the senate denouncing Agrippina and Nero. The latter was decreed to be a public enemy, and both were hurried into imprisonment in separate islands.

In a.d. 30, the second son, Drusus, who had been used by Seianus as a tool against his brother, was condemned by a similar decree, and imprisoned at Rome in a dungeon of the Palatium: also Asinius Gallus, while on a visit to Tiberius at Capreae, was arrested and sent back to Rome and there kept in custody.

At the beginning of a.d. 31, Seianus appeared to have reached the height of power. He was colleague with Tiberius in the consulship, and to some extent in the ‘proconsulare imperium,’ had also the dignity of pontiff, and was betrothed either to the widow or the daughter of Drusus. Also in this year Tiberius had so far overcome his hesitation as to order the execution or compel the suicide of Nero. Yet Drusus and Agrippina were living on; Gaius was still in favour and generally regarded as the heir; and these circumstances, added to sundry indications of coldness or suspicion on the part of Tiberius, appear to have precipitated Seianus into a plot to secure his end by assassination.