TODAY | March 07, 2014
>>> night. now her parents are hoping by speaking out about their experiences, other families will be able to avoid the tragedy they went through. here's andrea canning.
>> reporter: the future looked promising for skylar, she was 16, smart and beautiful. but one night, in the summer of 2012 , she disappeared from her home in morgantown, west virginia . her parents mary and dave were beside themselves.
>> mary told me, she said, something's wrong, dave . something's bad wrong.
>> skylar's best friends were the last people known to have seen her that night. igniting rumors about what they knew says skylar's classmate morgan lawrence.
>> between twitter and instagram and facebook , you just see things start to get ridiculously out of control.
>> the one thing that stood out to me is that people, teens were tweeting sheila and rachel had to know something.
>> they have written a new book about skylar's case.
>> she and rachel were dissing skylar when she wasn't around and putting her down and that kind of thing.
>> police and skylar's family came to believe sheila and rachel knew more than they were saying.
>> i started posting on facebook , things like karma. we were pushing the girls to get them to say what happened.
>> finally under pressure from skylar's family and the authorities, rachel confessed that she and sheila stabbed and killed skylar.
>> did you ask rachel why? why did you two do this?
>> she would not give us any specific detail. other than they just didn't like her.
>> sheila pleaded guilty to first-degree murder and was sentenced to life in prison . rachel pleaded guilty to second-degree murder. at a hearing just last week, she was sentenced to 30 years behind bars. she apologized directly to dave and mary .
>> there's not even words to describe the guilt and remorse i feel each day for what i've done.
>> now skylar's parents are trying to bring something positive out of their terrible experience. they helped pass skylar's law to expand amber alerts for teens and they urge parents to do everything they can to know their teens and their friends better.
>> my number one advice would be don't worry about being nosey. you know, get in their face, know what they're putting out there on facebook . i would be in her face so bad right now if she was still here.
>> for "today," nbc news, morgantown, west virginia .
>> and you can see more of skylar's story on "dateline" tonight at 8:00/7:00 central time right here on nbc.