True, but unless you see the fire, a professional reporter gives himself a little doubt regarding the character in a piece.
I think he was pissed because Petar was evasive and probably flippant in the interview, so he paid the price. Some small town reporters have a sort of God complex, and expect everyone to bend over backward for them. Petar was probably sharp, in a hurry and irritable with him instead of dealing with him professionally.
It's open question still whether his irritation on someone digging was because he had something significant to hide, or whether it was another reason.