Zach Mauer is the abusive ex-husband of Rowan Mauer. He killed their baby son Emerson when he attacked Rowan, causing her to bump into Emerson, who fell down the stairs and died. Rowan buried Emerson and kidnapped another boy, Kevin Brown, to be raised as Emerson.
When the real Emerson was found buried in Rowan’s backyard, Rowan admitted the truth and was arrested for kidnapping. (L&O: "Missing")