when her trial finally rolls around in 2017, prosecutor colleen barnett has a challenge.andy ended decades of apparently happy marriage with that kitchen knife. barnett admits, she can't explain why sandy did it. >> in texas we don't have to prove a motive. i thought that i was going to be able to prove that she did what she did, that i wouldn't be able to prove motive. >> that raised red flags for me. this woman doesn't seem like she could be a killer, but the prosecutor says she is the killer, but we don't know why. >> reporter: barnett says a possible motive came to her in the middle of the trial. >> and it wasn't until the jehovah's witnesses testified from the defense and i learned a little bit about the religion that i thought possibly that might have been part of it. >> reporter: defense witnesses say the melgars appeared to have had a good marriage. nevertheless, barnett conjures a theory. sandy wants a divorce, but afraid of being shunned by her fellow jehovah's witnesses, decides the easier option is murder. >> i don't know what her motive was. sometimes people

according to prosecutor colleen barnett of the harris county district attorney's office, that was becauservations about the crime scene. >> officers have investigated burglaries and robberies for many years before they get in homicide. and they thought that the scene looked kind of suspicious. >> so what was off kilter with this one? >> the fact that it didn't look burglarized. >> the drawers were open, i think, right? >> they were open a little bit, but nothing was tossed. >> reporter: but what about those items lizz noticed were missing from the house? and that backpack of apparent burglar's loot she found in the garage? well, detectives took careful note of many other items, pricey things that had been left untouched. >> cameras, the bicycle. there was some painting equipment. there were things that were easy to take that weren't taken. and then the stuff that was taken and put in the backpack was left in the garage. doesn't make sense. >> reporter: the home, to investigators, showed no sign of forced entry. that open garage door the only possible way in for an intruder. but law enforc