so detective zach johnson's attention turns to sonnier's girlfriend, richelle shetina. >> meeting with richelle that very first time i don't know a lot about her. >> did you ever have a time where you suspected that been involved in dr. sonnier's death? >> you always have to consider people the closest because 90% of the time it's going to be the people that are closest to our victims that committed crime against them. >> reporter: the police take richelle in for questioning. she's on the hot seat, and she starts naming names. >> there's just, you know, some girls that he dated. and joseph would get these horrible text messages, you know, calling him [ bleep ]. you know, you're such a [ bleep ], whatever. >> reporter: when "20/20" returns. hey, you forgot the milk! that's lactaid. right. 100% real milk, just without the lactose. so you can drink all you want... ...with no discomfort? exactly. here, try some... mmm, it is real milk. see? delicious. hoof bump! oh. right here girl, boom lactaid . 100% real milk. no discomfort not all wrinkles are the same. some wrinkles, like the vertical

and the first person homicide detective zach johnson wants to interview is richelle shetina, sonnier's current girlfriend. he brings her in for questioning. >> how would you describe your relationship with dr. sonnier? >> he's -- he's the love of my life. >> meeting with richelle that very first time, i don't know a lot about her. there were a couple of things that were mentioned by her that struck me as odd. months leading up to the murder she feels like someone was watching her, following her to the gym. >> i had gone in to work out and i pulled into the parking lot and there was this big creepy-looking guy sitting there. and at the door, like he was looking for someone or watching someone. >> reporter: she also thinks someone was hiding in sonnier's back yard, spying on the couple one evening after dance class. >> it was dark outside, and joseph and i both saw a flash from the outside. we laughed and kind of joked about, you know, maybe somebody was following us. >> reporter: detective johnson asks richelle if she knows anyone who might have stalked the doctor or held a grudge again

as do richelle shetina and those ex-girlfriends. detective johnson is stumped. >> there's something not right here. this is a huge r: until johnson gets the call all detectives pray for. it's from a tipster who offers a new name that could crack the case. >> his name's dave shepard. >> reporter: but who is dave shepard? stay with us. , stephanie. we're, like, so goth. we're, like, goth goth. honey? i'm dying my hair, mom. hair dye? no, not in my bathroom! honey, just let me in!. no. tiffany! no.ptiffany! it's just purple. teenage daughter? get scrubbing bubbles. kill 99.9% of germs and destroy dirt and grime.bbing bubbles disinfecting cleaners for 100% problem solved. we work hard, so you don't have to. sc johnson, a family company. bleeding gums? you may think it's a result of brushing too hard. it's not. it's a sign of early gum disease which you listerine added to your brushing routine listerine kills up to 99.9% of germs and helps reverse early gum disease in just two weeks. power to your mouth also try listerine floss. not much

joseph sonnier stunned everyone including his girlfriend richelle shetina. authorities found heas an innocent victim of a disturbing scheme in a case of jealousy and obsession spinning out of control. >> i was in shock. i was devastated. >> reporter: she was an unwitting part of a bizarre love triangle. >> evil followed me straight to joseph's door. >> reporter: and this morning richelle shetina girlfriend of wealthy prominent texas doctor dr. joseph sonnier found stabbed lubbock home is speaking exclusively to abc. about the man convicted in his her ex-boyfriend. dr. michael dixon. >> there was an ugliness from him to me. >> reporter: it's a tale of jealousy, obsession and revenge. dixon prosecutors say was in love with shetina, desperate to split up her relationship and hired his friend ex-con dave shepard to kill sonnier. it's a claim dixon's lawyers vigorously deny. >> mike dixon never wanted that to happen. >> reporter: days afterwards shepard confses. >> i check his pulse in his neck. he's dead. >> reporter: implicating his buddy. >> he wanted him hurt

. >> reporter: and this morning richelle shetina girlfriend of wealthy prominent texas doctor dr. joseph sonnier found stabbed and shot to death in his upscale lubbock home is speaking emotionally to xclusive exclusively to abc. her ex-boyfriend. dr. michael dixon. >> there was an ugliness from him to me. >> reporter: it's a tale of jealousy, obsession and revenge. love with shetina, desperate to split up her relationship and hired his friend ex-con dave shepherd to kill sonnier. it's a claim dixon's lawyers investigate vigorously deny. >> mike dixon never wanted that to happen. >> reporter: days afterwards shepherd confesses. >> i check his pulse in his neck. he's dead. >> reporter: implicating his buddy. >> he wanted him hurt and then it gradually developed into having him killed. >> reporter: shepard saying dixon paid him three bars of silver and a box of cubans for the hit. the monk of his funeral dixon arrested charged with capital murder. shetina says she'll never forget that phone call. >> my knees literally collapsed and i just kind of sank to the floor. and i cried and i