Broken by Karin Slaughter

Broken is one of the early Will Trent books. Will is sent to Grant County over Thanksgiving weekend to help Sara Linton with a case. His partner, Faith, is ready to deliver her baby, so she's mostly helping Will over the phone. A college student named Allison is found dead in a lake. At first, the cops think it's suicide then realize she was murdered.   A developmentally disabled young man is arrested for the killing, though Sara (who was his pediatrician for years) is certain that he didn't do it. Will is working with the local cops on the case, but they resent his presence and he's certain they're tampering with the evidence and keeping things from him, which is making his job harder.

At the same time, Sara and Will (who first met working on an earlier case) are drawing closer to each other, but there's still the problem of his estranged wife Angie.