In theory, the highest and last level of accountability is the voters. The system isn't perfect, but it's still democratic enough that this sort of thing couldn't happen without the approval or at least indifference of a big chunk of voters.
I don't think you can call voters a level of accountability given how trivial it is for voters to be lied to. Like politicians lie constantly, not even in the weasely "well I said this, not this" sort of thing, I mean they just lie. They tell easily dis-proven lies, on camera, with no accountability from the media or any kind of overseeing entity.
It’s easy to detect their lies. The problem is that the voters don’t want to know they’re being lied to.