Bounty Hunters
Remember the move “Ransom” (1956, remade in 1977). The twist is when the father of the kidnapped child goes on television to announce that the money won’t be paid as ransom, but as a reward to whomever turns in the kidnapper.
I think that if enough agencies get together, this might be an alternative strategy to tracking down these bastards. I can’t imagine they spend their time entirley in the company of wholesome and upright humans, and sooner or later someone will value the reward more than their association with them.