It's entirely possible that the father's actions in that situation were not logical. But leaving his critically injured and helpless son lying in the dark crime scene, not knowing where the attackers were, seems so unlikely it suggests the son was not alive when the father arrived. It makes me consider that the sheriff wants the perpetrators to believe a victim lived long enough to provide information critical to identifying them.
The sheriff 's hedge about one or more assailants suggests the son was not alive or was unable to communicate when his father arrived also. If the son could speak, the sheriff would have definitive information. The father would have been asking what happened when he discovered his son injured and his friends dead. If his son was able to speak, he'd have said the name(s) of the attackers if he recognized him/them. If it was a random attack, the pronouns would indicate whether they'd been attacked by one person or more than one.
All my opinion only.