There really are a variety of different things the PCs could do and technically they're all right. Rapport is your standard convincing people without lying skill, but they could go with deceit (I.E. telling a lie that makes the truth easier to get to/convince), scholarship (I.E. "It's the truth and I can prove it... With SCIENCE!), contacts ("This buddy of mine is in the police force"), resources ("I've got these photos"), or even intimidation ("You'd better believe me!"). Admittedly some of these things would work better as declarations, maneuvers, or blocks, but they all have a potential part in social conflict.