is there a chance your players can read this?
If you give us some story details we can think of better ways to gets around/bend/ or out right break "The Rules" so you can mess with them
He's some ideas I have
Tell PC the secret (kinda) ,that your guy is a Mole. BUT mislead this person so that he thinks the mole is a triple agent (Works for the Cia, pretends to double cross for KBG, but is actually working for CIA) . Or have it so the Mole has something so damning on him the player wouldnt dare tell or risk getting killed/beheaded/whatever
Or make it that everytime they get close to figuring it out, Ninjas attack.
If the Mole is a Wizard, make it so that he gives your groups Magical items (but the catch they dont know is, they also let him Track/listen to the group)
Show that your Mole is being "attacked" by the Bad Guy too. Take away any doubt.
Example.
Your Mole is the DunkinDoughnut Man, your bad guy is a Wizard. You see the Wizard Blowing up the Dunkin Doughnut mans Donut Shop. Tragic right? No turns out, the Doughnut man wanted to get out of the doughnut biz, teamed up with the wizard and split the insurance money. Your group could find this out by doing arcane and normal investigation on the scene (The Wizard used fire magic, but there were also a strange amount of gas cans in the shop)
You could also use academics skill to do research on the insurance policy of the doughnut man.
Will a group do that? I doubt it. That sounds boring. But its an option