I.on [美 ɑn, ɔn, 英 ɒn]介词on often appears as the second element of certain verb structures in English (count on, lay on, sign on, etc). For translations, see the relevant verb entry (count, lay, sign, etc).
It is an example of the prisoners' dilemma -- albeit simplified -- where two prisoners held separately end up mutually better off if they refuse to squeal on each other.
These agencies largely rely on the possibility that, by chance, a conscience-stricken employee of the private contractor will squeal on the thievery of public funds.
