In the company of darkness (1993)

After young policewoman Gina Pulasky succeeds in handling a domestic fight particularly well, she's added to a small team of detectives assigned to a case of gruesome child killings. She proves herself to be a worthwhile reinforcement by tracking down the prime suspect, a disturbingly smart young man named Kyle Timler. Much to the horror of her team mate and lover Will McCaid, Pulasky goes undercover to infiltrate in Timler's life and trying to get as close to him as necessary to make him confess his crimes without even noticing it. A dangerous mission involving a process of growing identification with the killer, something that doesn't leave Pulasky unscathed.