Dateline | January 17, 2014
>>> in the winter of 2012 , as the seneca county district attorney was preparing to prosecute carl carlson for the murder of his son levi , new york investigators had become deeply interested in that long-ago california fire that killed levi 's mother, christina . they were suspicious about carlson 's behavior that day. christina 's family was more than suspicious. they were certain carl killed christina . and now new york state investigator jeff arnold thought so, too. why did you take such an interest in tries tina's death when you're here in new york state -- that case is in california --
>> it became personal because she's just a great human being who had everything to to see look forward in life.
>> investigator arnold was asked it help out early in the carlson investigation and took a particular interest in christina and the fire that killed her.
>> just imagine yourself, you're in this bathroom taking a bath, and you smell smoke. and it's coming through your door, and there's this inferno sitting outside the door. you turn to the only window in the bathroom, and it's covered with a sheet of plywood.
>> investigator arnold says that pretty much everything carl carl carlson has ever said about the fire is a lie. beginning with the kerosene that fueled it.
>> he says that the kerosene got spilled in the house on sunday.
>> that was a few days before the fire. a california arson investigator found carpet stains that showed a second kerosene spill happened moments before the fire started. investigator arnold said that could only mean one thing --
>> carl poured the kerosene all over the clothes and all over the floor and ignited it. you know, within seconds after pouring it.
>> back in 1991 , carlson told law enforcement it could have been a dryer, a heater, or a utility light that touched off the blaze.
>> none of these happened.
>> arnold learned the arson investigator could find no accidental ignition source. the investigator's conclusion -- the fire was deliberately set. the coroner's report said christina died of smoke inhalation as opposed to burns, raising the question didn't carlson have time to save his wife.
>> he's standing on the porch within a foot or two from this plywood-covered window where christine is in dire need of his help. he doesn't even reach for one tool.
>> arnold knew that christina 's family had always wondered about that window and about some other things that happened after she died. colette told police something christina 's little daughter erin said right after the fire.
>> she said, "i heard mommy calling for daddy, but daddy just drove away." so within an hour, you know, i'm hearing my niece telling me, hey, mom was alive when he drove away.
>> just four days after the fire, carl carlson abruptly left california with his three kids. erin , cateskatie, and levi . sources tell us once carlson was back in new york , investigators begged their bosses to let them follow him for questioning. but they say they were told no. the county couldn't afford it.
>> the investigators wanted to go to new york , calaveras county wouldn't let them spend the money.
>> just weeks after they left, colette learned that carl had taken out a $200,000 insurance policy on christina 's life. and the policy paid off. colette came to believe that investigator arnold agrees that carlson killed christina for the same reason he killed levi -- money.
>> hey --
>> two decades after christina 's death, as carl carlson duelled with detectives in that seneca county interrogation room, he suddenly had to face questions about that long-ago fire from a well-prepared jeff arnold .
>> he was telling me about the fire with your first wife, that concerned him.
>> arnold was looking for inconsistencies, lies. he asked carlson about the window.
>> so what is it that you poured -- a --
>> months before.
>> months before?
>> months before.
>> we have a witness that shows up at the house on the 31st of december, one day prior to the fire. and that window was not boarded up.
>> carlson also told arnold he suffered awful injuries as he rescued his children and that the fire was so intense he couldn't save christina .
>> what happened is when the -- when i broke the window, i got hit with a fireball. it hit me right in the face. blew me off the front porch , burned my eyelids, took -- burned skin off my face. burned my mustache, any hairs.
>> did you get --
>> i don't know.
>> arnold had evidence that carlson escaped with only small burn. now after listening carl talk about the fire, the window, and the rescue of his children, he fired back.
>> your wife's in a boarded up room and you hear her calling from --
>> it wasn't calling. it was screaming.
>> screaming your name. and yet you're able to get blown off the porch, open your eyes , your son's uninjured, you're able to grab your uninjured son when this explosion took place without a mark on him, take him out of the house, run around, miraculously save your two daughters and let your wife -- and let your wife perish in this fire --
>> i didn't let her --
>> with no time to rip the board off the house and get into that bathroom and help her?
>> by that time the fire was all the way around. what am i going to do?
>> carlson insisted his first wife's death of a tragic accident. that's the same thing he was saying about levi 's death on. november 6th , 2013 , the very day his trial in levi 's case was supposed to begin, he surprised nearly everyone and took a plea.
>> karl karlson pled guilty to murder in the second degree in the seneca county court. taken responsibility for the death of his son levi .
>> it's called depraved indifference murder --
>> seneca county d.a. barry porsche said karlsen would be given 15 years to life. at the same news conference, levi 's sisters, erin and katie, thanked the investigation team. but then erin turned to the tragedy that shattered her childhood and changed her family forever.
>> we've been waiting just about 24 years now for some closure for things that transpired in california . you know, things that i will never be able to forget, things that my family will never be able to overcome.
>> erin 's aunt colette believes calaveras county has failed her sister and her family. she wants karlsen prosecuted for the murder of her sister. and she's keeping the pressure on in part with this facebook page devoted to christina and levi .
>> i think calaveras county needs to right the wrong.
>> and someone may be listening. the calaveras county d.a., barbara youck, wouldn't speak with us on camera but told us her office is investigating christina 's death as a homicide. she said she could not comment on the evidence but that they are committed to pursuing justice for christina if at all possible. the new york state investigators say they hope that's true and are ready to help.
>> you don't get a free pass because you commit a second murder. and you don't get a free pass because 20 years has passed. it's time that karl karlsen 's held accountable for what he did to christina karlsen in 1991 .
>> how do you want people to remember her?
>> if you go on the facebook page and read the comment business her, they all talk about how -- how she just had this incredible laugh. that's how people remember chris. that's the person that they'll never forget. what a wonderful, wonderful, sweet person chris was.
>> that's all for this edition of "dateline." we'll be back again sunday at 7:00, 6:00 central. and i'll see you tomorrow on "today." i'm lester holt . for all of us