A.J. Armstrong is accused of killing his parents as they slept in 2016, when A.J. was just 16 years old.
Arson expert Deborah Lynn took the stand to discuss that piece of evidence found on the second floor of the home. A.J. brought up the burned carpet during a taped police investigation after his parents' murders.
"I was playing with matches," he said to police. The tape of the interrogation was played in court Thursday.
A.J. said he lied when his dad, Antonio Armstrong, confronted him about the burn on the carpet. "My dad took that personally. You straight up to my face lied to me, but me and my dad, whenever we do have problems, we sit down and we solve it."
Lynn said she was asked by police to analyze the piece of carpet, a match and an empty plastic bottle collected from A.J.'s room. She said gasoline was found on the carpet and inside the bottle was a mixture of gasoline and rubbing alcohol.
During the defense's questioning, Lynn admits the items she tested were "stored in a way she didn't agree with."
She also said she has no idea when the carpet was burned or how old the burn mark was.
Later in the day, more forensic experts took the stand to discuss DNA and fingerprints from the scene.
Forensic experts said no DNA was found on a gun or its magazine but some DNA was found on the handgun's case, although it did not link back to A.J.
In addition, fingerprints were found inside a box of bullets but an expert said he "didn't think the prints were strong."
Prosecutors also wrapped up their questioning of HPD's lead detective, Sgt. Kenneth Daignault.
Earlier in the week, the defense appeared to suggest that the killer is actually A.J.'s older brother, Josh Armstrong.
Daignault followed up on that, telling jurors that he interviewed Josh's girlfriend because, "I found the defense was going to try and put the murders on Joshua Armstrong."
Daignault said that after he spoke with her, he had no reason to doubt that A.J. was the murder suspect.
A.J. has always maintained that an intruder shot and killed his parents.
The Armstrongs' alarm system has been heavily focused on in the case. Prosecutors say the alarm system never went off, despite A.J.'s claim of a masked intruder.
The defense focused on Daignault's check of the alarm system right after the murders and then again in 2018.
Attorneys argued Daignault tested the system by simply opening and closing doors and windows to hear a chime. They also hammered the fact that the alarm company didn't do official tests.
"You do not think access via the garage and the garage door is important?" the defense asked. Daignault says he didn't.
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