Dateline | April 03, 2013
>> reporter: how much do we reveal to the ones we love? do we keep our betrayals hidden behind closed doors? or do we hide in plain sight? masking our deeds with words?
>> text message says, i'm fine. just clearing my head.
>> reporter: that was kevin mengel for a time, clearing his head, he told his family and friends. facebook status, wants to be left alone .
>> maybe he just had enough and needed a vacation for himself.
>> reporter: but there was a trail of electronic messages that seemed to tell another story. a far more sinister one.
>> we were floored. it was all there for us to read. you can't make this stuff up.
>> reporter: which was fact and which was fiction?
>> these text messages don't even seem real, like they don't -- they just don't seem real.
>> reporter: long before kevin mengel had a cell phone or a facebook page, he was a high school guy in suburban philadelphia with a cute little sister to worry about.
>> kevin was the stereotypical older brother. he would follow me around and tell all his friends were not allowed to talk to me.
>> reporter: was he being protective?
>> very protective.
>> reporter: for as long as michelle can remember kevin mengel was a family man, a big brother who felt he needed to look after his younger siblings after their parents divorced.
>> kevin had a terrible time with it. and just always told us that he would never get a divorce, he would never get a divorce.
>> reporter: instead kevin grew up in search of perfect romance.
>> kevin always had girlfriends.
>> reporter: but he was always sort of a monogamous guy, when he was with a girlfriend, he was with a girl?
>> yes. and kevin was very committed to making sure that he stayed in a relationship, and he gave it his all.
>> reporter: kevin was 20 when he met the one, his forever girl. morgan , from a few towns away, where she graduated from an exclusive catholic girls school .
>> morgan was very, very charming. i mean, she was very sweet. and very attentive to me. always came over the house just to hang out with me.
>> reporter: outgoing and charismatic, morgan made an effort to win over kevin 's friends, too, mike and his fiancee, noel.
>> very nice, very upbeat, very friendly. always smiling.
>> reporter: and kevin ? well, he fell hard for morgan 's charm.
>> he just -- he loved her.
>> he was head over heels for her.
>> reporter: so kevin was about to get what he'd always wanted -- a wife and kids, but just not in that order. morgan gave birth to their first child a few months before he got married. still, theirs was no shotgun wedding . the young couple celebrated in traditional style.
>> kevin was on top of the world .
>> reporter: was the perfect picture, wasn't it?
>> it was the perfect picture.
>> reporter: kevin loved being a dad, and he and morgan had two more children, but his relationship with his own father was rocky, had been since his parents' divorce.
>> we're both stubborn and we're both pig headed. he didn't call me, i didn't call him, then it got into a little bit of a battle.
>> reporter: and years went by?
>> and years went by.
>> reporter: but by june 2010 kevin and his dad had patched things up, grown closer than ever. they made plans for the whole family to celebrate father's day together.
>> he was so excited. he said to me, been i can't wait to come out for father's day. i said i can't wait to have you. i love you and this is so wonderful.
>> he seemed to be at a peace that i'd not ever seen him at.
>> reporter: then the morning of the father's day party when it happened. sister called their mom to check in.
>> she didn't sound right. i said, are you okay? and she said, kevin left. she started crying hysterically. kevin left, kevin left. i said, mom, something's not right here. he left the three kids. he left morgan . i don't know where he went. he won't pick up his phone. he just sent me a text.
>> reporter: kevin leave his wife and kids? perhaps morgan put it best when she typed out their father's day regrets to kevin sr.
>> we received a text message from morgan saying, due to unforeseen circumstances, we'll not be able to be there today. happy father's day.
>> reporter: unforeseen circumstances? what exactly did that mean?
>> we all knew how excited he was for father's day. it would be the one day kevin would not miss, ever, no matter what happened.
>> reporter: no matter what happened. where had kevin mengel gone?