While walking the rooftops in search for the thief, the Assassin's attention turned towards a loud crowd that was gathering in front of the house of the local doctor; Pietro de Galencia. The Assassin moved closer to the house only to arrive and witness a gravely, tortured body of an old man being dragged out of the house. I later learned that his Pietro was in truth a Borgia Agent hired by Cesare Borgia. He was tasked to look for the secrets of the Assassins and if possible; to get his hands on them. This meant that Pietro was the one who stole Mario's chest from the Chambers.
Pietro moved to Monteriggioni a few years back claiming to be a doctor; though now I suspect that he was just a very good actor. He made friends with the citizens and managed to quickly gain their trust and friendship. Two days before he stole the chest, an old man with a broken hand visited Pietro. Whilst they were sharing their experiences of living in Monteriggioni, the old man confessed that his son was an Assassin acting in Roma. Pietro knocked out the old man and dragged him to the cellars where he tortured him for information. The old man revealed the location of one of the secret entrances; which eventually led Pietro to the chest.
As Pietro was about to escape the city, the old man somehow escaped from the cellar and alerted the citizens about Pietro's betrayal. Forced to think fast, Pietro summoned his henchman to the front of the church. But it was too late as the Assassin was already in pursuit of him. When Pietro's gaze met the Assassin's, he tried to flee but the Assassin caught up to him and killed him swiftly.
Unfortunately, the chest was no longer in his possession. The Assassin sprinted to Pietro's house but all that he was able to find in a hidden compartment in the wall was a stack of letters. Each of these letters contained the Borgia seal and were written by Cesare Borgia himself; referring to Pietro as an "agent".