It wouldn't surprise me if this hasn't been suggested already, but increase the reward by $1000 a day.

A free-market auction and game theory (this is pretty similar to the prisoner's dilemma game) all rolled up in one! At some point, the money is big enough that someone figures that one of their fellow conspirators is going to take the offer, so why not be the person who gets the cash? As long as one's reputation is going to take a hit, might as well get paid for it, right?