Avoidable Arguement

The section i found most interesting in this work is the dialogue between Oedipus and Teiresias towards the beginning of the play.  Starting on page eight line 300, Oedipus implores Teiresias, the blind prophet, to tell him who in the land is responsible for the curse put upon them so that they may drive them out and be well again.  I fully expected Teiresias to tell Oedipus the culprit without hesitation because he would have been helping them but instead he refuses to tell him who it is.  He says instead,"Let me go home. It will be easiest for us both to bear our several destinies to the end if you will follow my advice".  From this point until page twelve Oedipus and Teresias argue and insult each other.  During the arguement Oedipus taunts Teiresias about being blind and then finally Teiresias says who the cause of trouble is: Oedipus.  Of course after saying this (and by Oedipus' request, repeting himself) Oedipus accuses him of saying this in anger and not in truth.  Eventually Teiresias leaves and Oedipus questions Creon about why he brought Teresias to prophesy if he was going to tell false truths.  My question here is why did'nt Teiresias say who the "land's pollution" was in the beginning when he was first questioned about it?  If it really was Oedipus who was the problem it would have been easier believed if said straight off the bat than during the arguement.  I can see the point in Teiresias not saying it at first because he knew Oedipus would be upset but i feel it would have gone over better.