If you have to decide one or the other, then feel free to pick internal or external and defend your position. If you are allowed to say that the internal and external conflicts are evenly weighted, then go with that.
The external conflict is definitely present and could be considered the main conflict of the story. Kevin fighting for his life against Richard Slater's riddles and bomb threats is definitely a central conflict to the story.
As for an internal conflict, Kevin's emotional and mental turmoil over his childhood and how Belinda treated him during those years is also central to the story line. A second inner conflict focuses on Kevin's examination of some previous, hidden sin that Slater seems to be obsessed with. Kevin believes that it involves events that happened when he was eleven years old, and his inner turmoil leads him to confess that sin to the media.