By this time the enmity between Drusus and Sejanus had reached a critical point: Drusus had struck the prefect with his fist, and openly lamented that "a stranger was invited to assist in the government while the emperor's son was alive". According to Levick, there is nothing to suggest bad relations between the two before this point, especially not as far back as 14, during their expedition to Pannonia. She says, however, it may have been the clash between theatrical "claqueurs" and the Praetorian Guard in 15 that began the quarrel, but not enough is known to say for certain. Of more concern to Drusus was, perhaps, that in 20 his father mentioned Sejanus to the Senate in terms that treated him as his partner in power. For Drusus, the heir of that exact position, this was a legitimate cause for concern. At the time, Tiberius was in his sixties, so there was a realistic possibility of Drusus succeeding his father in the near future. In the account of Tacitus, Sejanus began plotting against Drusus in secrecy to secure his position.