An expected 3-day preliminary hearing is now underway, for accused child killer Nathan Sloop, 34.

Prosecutors allege Sloop -- and the boy's mother, Stephanie Sloop -- killed Ethan Stacy in 2010.

Graphic testimony was offered on the opening day of the hearing, which will determine whether Sloop stands trial for the death.

Defense attorneys argue the boy died from a medication overdose.

Prosecutors say Sloop killed the boy, and then disfigured his face with a hammer in an effort to conceal his identification.