They go ahead and bring Jonathan in for questioning the next day, and they call him to come in after work.
In the meantime, Sam brings in Terence Andrews, who is a gigantic turd.
He brings a lawyer, acts super important, and mostly dodges all the questions, though he does agree to participate in a voice line-up.
That night, Cassie and Rob interrogate Devlin, bringing up Sandra's rape and the fact that the missing kids saw them.
This escalates until Cassie pretty much accuses Jonathan of killing the kids in the woods in 1984.
He says that they're "out of [their] bloody minds" (14.101), though, and steers the conversation back to Katy.
They suggest that Mrs. Devlin was poisoning Katy, but Jonathan doesn't believe that would happen.
Things get more and more heated until Rob lunges at Devlin. Cassie plays good cop and tries to restrain him, but he's not playing this time—he's furious and angry, and they have a real struggle, in which he accidentally smacks Cassie in the mouth.
They let Jonathan go.
Cassie is furious at Rob, and he's mad at himself for getting carried away and hurting her.