Titus Livius (Livy), The History of Rome, Book 2
Benjamin Oliver Foster, Ph.D., Ed.

4. The brothers Vitellii and Aquilii were the first to be entrusted with the project. A sister of the Vitellii had married the consul Brutus, and there were sons of this marriage who were now young men, Titus and Tiberius; these were also admitted by their uncles to a share in the design. [2] There were besides several other young nobles taken into the secret, but their names are lost in antiquity. [3] The senate meantime had acquiesced in the opinion of those who were in favour of giving back the property. This very fact gave the agents of the exiles an excuse for lingering in the City, for the consuls had granted them time for obtaining vehicles with which to carry away the belongings of the royal family. [4] All this time they spent in consultation with the conspirators, whom they urged and at length persuaded to give them letters for the Tarquinii: for otherwise how could the princes be convinced that the statements of their agents regarding matters of such importance were to be relied on? [5] These letters, being given as a pledge of sincerity, furnished clear proof of the crime, For on the eve of the envoys' setting out to rejoin their masters it happened that they were dining at the house of the Vitellii, where the conspirators, having dismissed all witnesses, had much talk together, naturally enough, about their [p. 231]new design. This conversation one of the slaves¹ overheard. [6] He had for some time perceived what was in the wind, but was waiting for the opportunity which the delivery of the letters to the envoys would provide, that their seizure might make good his accusation. When he saw that the letters had been given, he laid the matter before the consuls. [7] The consuls left their houses, arrested the agents and the conspirators, and, without making any disturbance, completely crushed the plot, being especially careful not to lose the letters. The traitors were thrown into prison forthwith. As for the envoys, it was uncertain for a little while what would be done with them, but, notwithstanding they appeared to have deserved no less than to be treated as enemies, the law of nations nevertheless prevailed.