Armstrong Jr. showed no emotion in the courtroom but his wife Kate Armstrong was crying after the verdict was read.
"He's devastated but he's one of the strongest young men I've ever known," defense attorney Rick DeToto said.
Jurors in the defendant's third trial deliberated about 10.5 hours after hearing 11 days of testimony with 31 witnesses.
He was 16 years old when his parents were murdered so he was automatically sentenced to life with the possibility of parole after 40 years instead of the death penalty.
Dozens of family members, including Armstrong's sister, grandparents, aunts, uncles, and cousins attended the trial to show support. His sister Kayra, who was 12 when their parents were killed, and his paternal grandmother Kay Winston testified for the defense.
"His family is amazing. Obviously, they're devastated right now. This is a difficult time for them, but they will rally and help A.J. and his young son and his wife get through this," DeToto said.
He said the case will be appealed.
"We tried to give him hope, you know, it's a long road," defense attorney Chris Collings said. "Everybody's in shock and disbelief and it's going to take some time to process everything that's just occurred."
Harris County District Attorney Kim Ogg said her team, led by prosecutors John Jordan and Ryan Trask, spent countless hours to bring justice on behalf of the victims.
"In trying to speak for those who no longer had a voice to defend themselves or say what happened, our job was to find justice for them," Ogg said in a news conference. "Antonio Sr. and Dawn Armstrong died because they were trying to be good parents because they wanted their children to do right -- not to lie, to work to be law-abiding, contributing adults."
Throughout the trial, the victim's oldest son, Josh Armstrong, was painted as a more likely suspect by the defense because of his history of mental illness. But prosecutors said those issues didn't start until after the murders.
"There's somebody else we were in the courtroom defending and that was Josh Armstrong, and we passionately believe that it was inexcusable to drag his name through this courtroom in order to get this guilty defendant off," Jordan said.
Prosecutors said the jury was "confident in its decision" without needing the blood evidence that's now the focus of a federal lawsuit Armstrong Jr. filed against the City of Houston, accusing HPD of planting it.
Trask said the main factors in their decision were the shot Armstrong Jr. fired through the floor of his bedroom with the murder weapon, the fire he set outside his parents' bedroom, the 911 call and the inconsistencies in his statements.
Before the case went to the jury, prosecutors and defense attorneys spent four hours of sometimes heated closing arguments trying to convince jurors that Armstrong Jr. was or wasn’t the killer. Both sides recapped mountains of evidence, including the 911 call, the defendant's interview with homicide detectives, security alarm records, cell phone records and text messages.
Prosecutors pointed to three key events before Antonio Sr. and Dawn Armstrong were killed while they slept. About a week before the July 29, 2016 killings, Armstrong Jr. shot his father’s gun through the floor of his bedroom through a pillow and blanket.
"He shot the murder weapon in his room. Who does that?" prosecutor John Jordan asked loudly. "Purely coincidental? It’s ridiculous. Ridiculous."
Jordan said the then-16-year-old initially lied to detectives and said he did it because his friend had never heard a gun go off but later admitted he was alone at the time.
Forty-eight hours before the deaths, Jordan said Armstrong Jr. poured gasoline into a bottle of rubbing alcohol and set a fire outside his parents’ bedroom. He said Armstrong also searched how to make a car bomb on his iPad.
Jordan brought up Armstrong Jr.’s claim that he saw a 6-foot-tall, masked intruder after hearing gunshots in his parents’ room. The prosecutor pointed out that he didn’t mention an intruder on the 911 call or during the first few hours of his interview with detectives.
Also, during that interview, Jordan told jurors that Armstrong Jr. never asked about his mom or showed any emotion when told she was dead.
To counter the defense’s claims that the victim’s oldest son, Josh Armstrong, was a more likely killer, prosecutors called on his longtime girlfriend, Hannah Pilon, who testified that his mental health issues didn’t start until after his parents were killed.
While Josh was also having issues in school and smoking marijuana, jurors saw dozens of text messages with his mother that appeared to show a loving relationship. She sometimes confided in him about her frustrations with his younger brother.
Armstrong Jr. had been in trouble with his parents for getting kicked out of Kinkaid High School, failing in school and smoking marijuana.
Jordan showed cell phone records that showed Armstrong Jr.’s phone was being used just before the early-morning killings, beginning at 1:09 a.m. Jordan said the cell phone and motion sensors chronicled Armstrong Jr. moving around the house until 1:40 a.m. when he called 911.
Finally, he pointed to alarm records that he said proved the victims and their two youngest children were the only ones in the home. "The alarm records show nobody entered the house that night, period, end of discussion."
Throughout the trial, jurors heard from 31 witnesses, including the defendant’s sister Kayra Armstrong, who was 12 when her parents were killed, and his paternal grandmother Kay Winston.
One family member who hasn't attended any of the trials was again the focus of much of the testimony. The defense has tried to paint the Armstrongs' oldest son Josh Armstrong as an alternate suspect. He has a history of severe mental health issues although there were no medical records of such issues before his parents' deaths.
Both sides brought on forensics psychiatrists to go over thousands of pages of medical records documenting Josh Armstrong’s downward spiral in the months and years after his parents were killed, beginning on Dec. 19, 2016.
The defense argued that Josh Armstrong showed signs of paranoia and schizophrenia before their deaths. They point to testimony from the sister and grandmother who said Josh Armstrong “was different” when he moved back home from Blinn College weeks before the killings.
Both women testified that he neglected basic hygiene, would stare off into space and spend hours in the bathroom talking to himself. They said he was smoking a lot of marijuana and was kicked out of his parents’ house after throwing a party while they were out of town.
"Both doctors (for defense and prosecution) agreed and testified that marijuana can serve as a trigger for psychotic episodes," defense attorney Chris Collings told the jury.
He pointed out that Josh Armstrong thought he was both God and the devil during later stays at psychiatric hospitals and once told a doctor "I witnessed my parents’ murders."
Collings referred to Kay Winston’s testimony that Josh Armstrong once lit a towel on fire and put it in the oven when he was living with her after the killings. She and Kayra Armstrong both testified they were afraid of him as his issues became more severe.
The defense also cast doubt on the accuracy of alarm records and blood spatter evidence and the lack of DNA evidence.
Collings told jurors: "There is nothing, absolutely nothing that proves AJ was in the bedroom where his parents were shot to death."
"That is reasonable doubt all day long," defense attorney Rick DeToto said.
Armstrongs found dead in 2016
On July 29, 2016, investigators said Armstrong Jr. shot his parents at close range while they slept inside the Bellaire-area home.
Armstrong Jr. called 911 at 1:40 a.m. and told dispatchers he heard gunshots coming from his parent's room. He said his then-12-year-old sister was sleeping downstairs.
Hours after the shootings, Armstrong Jr. blamed a masked intruder, but investigators said they found no evidence of forced entry into the house.
Dawn Armstrong was shot twice in her head and Armstrong Sr. was shot once. They said both had pillows over their heads. Dawn Armstrong was pronounced dead at the scene while Armstrong Sr. was taken to a hospital where he later died. Both were 42.
HPD Sgt. J.P. Horelica said after discovering there was no forced entry and a bullet hole in the ceiling of the Armstrongs' bedroom, the focus shifted to Armstrong Jr. as a suspect.
Prosecutors said a .22-caliber pistol was found on the kitchen counter along with a note that read, "I have been watching you for a long time. Come get me.”
Armstrong Sr. was a motivational speaker. He and his wife owned 1st Class Training in Bellaire. Armstrong Sr. played football for Texas A&M and the Miami Dolphins and coached both of his sons when they were younger. Armstrong Sr. was also an associate pastor, according to police. Dawn Armstrong's Facebook page said they "serve in ministry together."
First capital murder trial
Armstrong Jr.'s first murder trial began on April 2, 2019. He was tried as an adult and faced life in prison if found guilty.
During the first trial, Armstrong Jr. entered a not-guilty plea before opening statements. The initial trial ended in a mistrial on April 26, 2019, when jurors weren't able to reach a unanimous decision in the case.
Second capital murder trial
Armstrong Jr.'s second trial was supposed to begin in October 2019 but was postponed to January 2020. Days before it was set to begin, it was postponed yet again after more than 30 motions were filed in the case.
A judge declared a mistrial in the second capital murder trial of Armstrong Jr. in October 2022 after jurors deliberated for nearly 18 hours but couldn't agree on a unanimous verdict.
Defense attorneys said then that eight jurors believed Armstrong Jr. was innocent and four thought he was guilty. In his first trial, it was the other way around with eight jurors believing he was guilty.
KHOU 11 spoke with a juror in the second trial who didn't want to be identified. They said doubt created by the defense led to a hung jury.