Titus Livius (Livy), The History of Rome, Book 2
Rev. Canon Roberts, Ed.

4. The project was at first entrusted to the brothers Vitellii and Aquilii. The sister of the Vitellii was married to the consul Brutus, and there were grown-up children from this marriage —Titus and Tiberius. Their, uncles took them into the conspiracy, there were others besides, whose names have been lost.

[2] In the meantime the opinion that the property ought to be restored was adopted by the majority of the senate, and this enabled the envoys to prolong their stay, as the consuls required time to provide vehicles for conveying the goods. [3] They employed their time in consultations with the conspirators, and they insisted on getting a letter, which they were to give to the Tarquins, for without such a guarantee, they argued, how could they be sure that their envoys had not brought back empty promises in a matter of such vast importance? [4] A letter was accordingly given as a pledge of good faith, and this it was that led to the discovery of the plot. [5] The day previous to the departure of the envoys they happened to be dining at the house of the Vitellii. After all who were not in the secret had left, the conspirators discussed many details respecting their projected treason, which were overheard by one of the slaves, who had previously [6??] suspected that something was afoot, but was waiting for the moment when the letter should be given, as its seizure would be a complete proof of the plot. When he found that it had been given, he disclosed the affair to the consuls. [7] They at once proceeded to arrest the envoys and the conspirators, and crushed the whole plot without exciting any alarm. Their first care was to secure the letter before it was destroyed. The traitors were forthwith thrown into prison; there was some hesitation in dealing with the envoys, and although they had evidently been guilty of a hostile act, the rights of international law were accorded them.