After he was read his rights, Armstrong was questioned by two detectives for just under an hour, then arrested and charged with capital murder.
Seven years and three trials later, Armstrong has now been found guilty of killing his parents.
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Prosecutors said after discussing the verdict with the jury, jurors told them it was Armstrong's own words from the 911 call and the interrogation interview that ultimately helped them decide the couple's son was the true killer.
Now, for the first time outside of the courtroom, you can listen to Armstrong's 53-minute police interrogation.
Full interrogation: ABC13 obtains AJ Armstrong's 2016 HPD interview
Jimmy Dodson, now retired, was one of two detectives in the room questioning Armstrong. For the first time, only on ABC13, Dodson is talking about that interview.
"(Armstrong was) emotionless, stone-faced," Dodson said. "And that's what made it so eerie, actually, because we've got all of this evidence pointing one direction, and that's at him. But, even when, during the interview, whenever we tell him that his mother's gone and she's not coming back, there's no emotion at all."
Within the first couple minutes, Dodson and HPD detective, Sgt. Kenneth Daignault, tried to understand the Armstrong family dynamic.
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"You get along with your mom? You get along with your dad?" Daignault asked Armstrong.
"Me and Dad are really close" Armstrong said. "Me, my brother and my sister - had our little issues with our mom, but it was never anything serious. Just family stuff. We were all really close, though."
That struck Dodson.
"For us, we're like, 'That's a weird answer,'" Dodson told ABC13. "We just told you that your mom's not here anymore. You know, and typically, even if you don't like your mother or something like that, you're still going to want to be positive. Right? That's grief, you know? But it wasn't there. She was immediately villainized."
Armstrong shot his mother, Dawn, twice in the head, then his father, Antonio Sr., the jury agreed.
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Armstrong has long claimed he is innocent and that he saw a "masked man" run from the house. He first told police about the intruder during this interrogation but didn't mention it in the 911 call he made six hours earlier after hearing shots.
"We knew that he never mentioned the masked intruder in the 911 call," Dodson said. "If you actually saw that, that's the first thing you would say when you call 911. 'There's a guy in my house, there's a murderer in my house! I heard gunshots!' That's not what he says."
"Both in the 911 call as well as his statement, there's just this void of emotion," said John Jordan, the Harris County assistant district attorney who prosecuted the case along with ADA Ryan Trask. "You know, 16-year-olds, they're all about emotion. Their hormones. They don't know filter. And he is just chill throughout."
Armstrong was sentenced to life in prison with the possibility of parole after 40 years. He'll be 63 years old at that time. He has filed for an appeal. The Armstrong family continues to support A.J. and believes he is innocent.
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