If you mean the 2nd roman emperor Tiberius Claudius Nero then it was either Caligula (LIKELY) or Gemellus (MAYBE)
You must remember that Tiberius almost certainly killed Caligula's father Germanicus, who was the most popular prince of the Julio-claudian royal family. Caligula spoke several times of nearly killing Tiberius with a dagger while he was asleep. Because Tiberius died in old age it was likely not a murder. Tiberius was absolutely obsessed with his own security, which is indicated in the fall of Sejanus. Tiberius expected to be killed, and fought it hard, he died old and himself said that scarcely had any king lived who had enjoyed such tranquil old age.

He knew well that Caligula was volatile, and this is why it is not an automatic assumption that Caligula was guilty.
Tiberius wrote in his diary regarding his dual successors Caligula and Gemellus

" I am nursing a Viper for the Roman people, Caligula will kill Gemellus, and then someone else will kill Caligula"

How correct the old man was!!

Cambraceres