Just after noon, prosecutors questioned the trace evidence lab manager at the Harris County Forensic Science Center, Jason Schroeder.
Schroeder revealed no gunshot residue was found on A.J. or his clothing, as well as on his sister, who was in their family home on the night of the murders, or his brother, who defense attorneys implied was allegedly involved.
Still, Schroeder said a lack of gunshot residue doesn't necessarily mean that anyone was not near a weapon.
Additional testimony Monday also came from a firearms expert and an investigator with the Harris County District Attorney's Office.
The firearms expert said shell casings found in A.J.'s bedroom matched the gun belonging to his father Antonio.
The district attorney investigator also testified about testing the alarm systems belonging to a house owned by the Armstrongs months after the murders.
After a lunch break, prosecutors resumed calling witnesses, including an interesting choice - Stephen Hill, who was A.J.'s football coach at The Kinkaid School. This was significant because Hill is the first witness to this point who knows A.J. and is testifying for the prosecution.
Asked about what stood out about A.J., Hill said, "His athleticism. He was a fun-loving kid." He added that A.J. was a standout player for the team.
He was also called upon to recall A.J.'s interactions with his parents, saying the teen's mom Dawn was harder on him than Antonio.
"He was special to me," Hill said of A.J. "I guess I treated him a bit different than I treated anyone else."
Over the course of the testimony, A.J. was seen having no reaction to Hill, including when the former Kinkaid coach cried on the stand.
"This is just a tough situation for me," Hill said.
In cross examination, the defense brought up A.J. leaving Kinkaid for Lamar High School. While the defense contended it was for financial reasons, Hill recalled Antonio telling him that A.J. was kicked out for bad grades.
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This segued into the second coach to testify on Monday - former Lamar football coach, Theatus Reagins, who was familiar with Antonio since they played against each other in high school.
While Reagins' testimony was brief and toward the end of the day, he recalled A.J. had a good attitude and treated his teammates, coaches and parents with respect.
Entering the new week of the trial, A.J.'s attorneys described the first four days as a long week of fighting.
"They have this theory that, because A.J. was not going to start at Lamar, he killed his parents," said defense attorney Rick Detoto. "That's ridiculous. That's their motive. They don't have a motive, so they are reaching for straws."
Before the trial resumed Monday, jurors heard nearly 20 hours of testimony, including from the officer who interviewed A.J. on the morning of his parents' deaths, to experts on DNA and fingerprints.
The experts say no prints were found on the gun used to kill A.J.'s parents or the note left by the killer.
As the trial enters its second week on Monday, jurors are expected to hear from the medical examiner, the Armstrongs' family and friends, and alarm experts.
A.J. claims an intruder killed his parents, but prosecutors say the home's alarm system never went off.
The trial could last three more weeks.
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