Help with regards to the pre-frontal cortex and its effect on behaviour?

In 1848, a railroad foreman named Phineas Gage was injured in an explosion, when a tamping iron penetrated his skull and caused brain damage; surprisingly, he survived. The rod was 1.25" in diameter and 3.5' long. After the accident he was able to speak, get himself up and down from a cart, and climb a flight of stairs to see the local physicians. Years later, the doctors reported that he physically recovered, but the balance between his intellectual facilities and animal propensities was destroyed. He was no longer mild-mannered, kind, and respectful; he grew obstinate, ill-tempered, and showed little self-restraint. Based on this story, what area(s) of the brain (and/or systems) do you think he injured and why?