>> reporter: the caller was the k g -- knoefels' 13-year-old daughter, megan. she said her mother, lisa, was being brutally stabbed, right before her eyes. >> stabbing my mom with a knife. >> who has a knife? >> i have a female yelling on the phone. somebody else has a knife. >> megan was screaming, yelling at somebody to stop. "you're killing her. stop, you're hurting her. why are you doing this?" >> oh, my god. my mom's going to die. >> reporter: in the background lisa can be heard, pleading for help. >> call 911! >> what is going on? >> oh, my god! >> it's really chilling to listen to. >> you're going to have to take a deep breath. i cannot understand you. >> my mom's going to die! >> reporter: first responding officer, randy mullenex, arrived at the knoefel home and found megan frantic at the front door. >> she's waving her hands. "you need to hurry. hurry, hurry." >> reporter: mullenex learned kevin knoefel was away on business leaving lisa, megan, hailey, and their foster daughter sabrina at home. >> worst case scenario was going through my head. what am

. >> when she was told about being placed with the knoefels, she was ecstatic. a happy home until it was ripped apart in a single night. >> oh, my god! my mom's going to die! >> reporter: lisa -- stabbed. >> the blade was actually bent. it takes a lot to bend a knife blade. >> reporter: the killer -- none other than sabrina. >> i couldn't believe it. >> reporter: a loving mom, murdered by the foster daughter she'd opened her home to. what no one could understand, was why? >> something else had to have been at play here. >> reporter: indeed, this case was about to get a lot more complicated. stories of a diabolical plot, brewing behind closed doors. >> this is really just betrayal on top of betrayal on top of betrayal, isn't it? >> reporter: and an obsession that would not be denied. >> it was like, "i want this, so this is how i'm going to get it." >> reporter: i'm lester holt and this is "dateline." here's josh mankiewicz with "deadly betrayal." >> reporter: there comes a time in most cops' careers when they see something so horrific, they'll do just about anyt

. >> when she was told about being placed with the knoefels, she was ecstatic.til it was ripped apart in a single night. >> oh, my god! my mom's going to die! >> reporter: lisa -- stabbed. >> the blade was actually bent. it takes a lot to bend a knife blade. >> reporter: the killer -- none other than sabrina. >> i couldn't believe it. >> reporter: a loving mom, murdered by the foster daughter she'd opened her home to. what no one could understand, was why? >> something else had to have been at play here. >> reporter: indeed, this case was about to get a lot more complicated. stories of a diabolical plot, brewing behind closed doors. >> this is really just betrayal on top of betrayal on top of betrayal, isn't it? >> reporter: and an obsession that would not be denied. >> it was like, "i want this, so this is how i'm going to get it." >> reporter: i'm lester holt and this is "dateline." here's josh mankiewicz with "deadly betrayal." >> reporter: there comes a time in most cops' careers when they see something so horrific, they'll do just about anything to forget i