How prosecutors convicted the son of slain Houston couple after 3 trials

After seven years of investigations, accusations and three trials, prosecutors say they have finally found justice for Antonio and Dawn Armstrong.

The former NFL player and his wife were found shot in the bedroom of their Houston home in 2016 while their children, AJ and Kayra, were inside.

For years AJ, who was 16 at the time, was identified by investigators as the lead suspect, but he maintained his innocence and had the support of his entire family.

"I can tell you now, nothing like that will come back on me," AJ Armstrong told investigators when they questioned him after the killings and said they had evidence against him.

AJ was charged with two counts of capital murder and tried twice—with both trials ending in mistrials.

However, this year, prosecutors discovered a key piece of evidence that was overlooked and a jury convicted AJ Armstrong.

An episode of "20/20" airing Dec. 15 at 9 p.m. ET and streaming the next day on Hulu takes a look at the homicides and the legal case, including interviews with key members of the prosecution and defense, Armstrong family members and AJ Armstrong prior to his third trial.

Antonio Armstrong, a former NFL player and later owner of several personal gyms, and his wife Dawn lived with AJ and Kayra in their southeast Houston house.

On July 29, 2016, police received a call from AJ Armstrong saying that he heard gunshots from his parents' room and when first responders arrived they found the couple in bed with gunshot wounds to their heads.

Dawn Armstrong was pronounced dead at the scene; Antonio Armstrong died later in the hospital.

AJ Armstrong was immediately taken by the police for questioning, and during that questioning he claimed he saw a masked intruder in the house.

"Yeah, like, I saw him running. Like I saw him running," AJ Armstrong told investigators during the interview.

Investigators questioned this story because they found no evidence of an intruder, and the alarm to the home was still armed when first responders arrived, investigators said. AJ Armstrong was the one who disarmed the alarm to let the officers in, according to court documents.

The officers also had questions about bizarre evidence found at the family home.

There was a bullet hole found originating from AJ’s third-floor bedroom and penetrating the ceiling of his parents’ room.

The teen initially said during his interview with police that he didn't touch his father’s gun, which was determined to have been the weapon that killed Dawn and Antonio, but when pressed by law enforcement, he admitted that he showed off the weapon to a friend weeks earlier and fired it.

Additionally, investigators at the scene noticed a burn mark on the carpet at the top of the stairs to the second floor. When detectives questioned AJ Armstrong about this, he claimed that a few weeks before the murders he had been playing with matches and accidentally dropped one.

AJ Armstrong went on to say that he initially lied to his father about how the burn mark occurred, but eventually admitted it and they talked it out.

According to audio of the interrogation, detectives made clear to AJ Armstrong that they believed his story wasn’t "adding up" and that he knew more than he was saying about how his parents ended up shot in their bed. Armstrong steadfastly maintained his innocence, insisting he had nothing to do with his parents’ murders.

After the interview, AJ Armstrong was booked as a juvenile on charges of capital murder, much to the shock of his family. Armstrong pleaded not guilty to the charges. His case was eventually transferred to adult court.

"It's just not right. To be accused just makes everything so much worse. It makes it so much harder for my family," AJ Armstrong told "20/20".

Authorities allowed AJ Armstrong to attend his parents' funeral in handcuffs.

Attorney Rick DeToto was hired to represent AJ Armstrong. DeToto told "20/20" that investigators found no physical evidence, like gunshot residue on AJ or fingerprints on the gun left at the scene, to link his client to the Armstrongs' killings.

A handwritten note was also found in the house that read "I have been watching for a long time," but authorities determined that AJ Armstrong’s fingerprints were not found on the paper.

AJ Armstrong's attorneys also brought up other instances that they say police discounted that could have shed light on the homicides, including a break-in at one of Antonio Armstrong's gyms just days after the murders.

Investigators looked into the break-in, but determined it was not connected to the Armstrongs' killings.

In April 2017, AJ Armstrong was released to house arrest after his family posted bail. Nearly two years later, in March 2019, his first trial began.

Prosecutors argued that AJ Armstrong smoked marijuana and had a tense relationship with his parents due to his drug use and failing grades. This, they argued, and the lies they had caught him in, drove him to kill his parents.

AJ Armstrong's attorneys, however, reiterated that investigators had no evidence that proved he shot his parents.

"They didn't find any gloves with any evidence that he had worn them or there was no bloody clothes in the washer or dryer," DeToto said.

Ultimately, the jury was deadlocked and a mistrial was declared in April 2019.

"Honestly, the first thing to my head was just like disappointment, but I always try and find the good somewhere in the situation," AJ Armstrong said in 2022. "It's obviously a lot better than being in jail for the rest of your life."

Following the mistrial, AJ, who was still living under house arrest, welcomed a son with his longtime girlfriend.

A second trial got underway in October 2022.

Ryan Trask, an assistant district attorney for Harris County who was one of the prosecutors in the second trial, told "20/20" that he was skeptical about AJ Armstrong's changing stories about his relationship with his family.

"It showed that he was an individual who had the capability of telling you one thing to your face and having a completely different plan behind your back," he said.

This time prosecutors presented jurors with new evidence extracted from AJ Armstrong’s cell phone from the night of the murders. In addition to who he was texting, prosecutors were now able to tell when Armstrong’s phone was plugged in, and unplugged and when the home screen was active.

According to court records, at 1:08 a.m. Armstrong’s cell phone was taken off the charger, which prosecutors suggested was when Armstrong was heading down from his room to murder his parents.

AJ Armstrong's defense attorneys continued to argue that there is no direct evidence that shows their client pulled the trigger.

For the second time, the jury was deadlocked, and the prosecution pushed for another retrial. AJ went back home on house arrest and married his longtime girlfriend.

"They would continue trying him. And it's why we felt that we had to get a not guilty or the saga would not end," DeToto said.

Prosecutors said they feared they only had one more chance at getting a conviction so they "went back to the drawing board."

"We'd be fools if we tried to try it the exact same way, so we decided we would bring the crime scene to the courtroom," John Jordan, a Harris County Assistant District Attorney who prosecuted the case, told "20/20."

In 2016, investigators looked at AJ Armstrong's clothes but found no blood.

Blood pattern analyst Sgt. Celestina Rossi was brought in by the prosecution to look over the physical evidence and she examined a visitor's badge sticker that was placed on AJ Armstrong's shirt when he was first questioned by police at the station.

Rossi told "20/20" that just before the third trial was going to start in July, she found what looked like "small reddish brown circular stains," on the backside of that sticker which preliminary tests indicated was blood. She took the evidence to the Houston Forensic Science Center for further analysis.

"They had results pretty quick, and I was told that it had been identified as a single source to Antonio Sr.," she told "20/20."

The trial, which already had a jury chosen, was delayed several weeks after prosecutors alerted DeToto about the discovery.

"After careful consultation with AJ, with his family, with lots of different lawyers, that we're just going to let it in because it looks so desperate, and it would add to our theory that there's no evidence," DeToto said.

Rossi, who took the stand during the third trial, told "20/20" that Antonio Armstrong was trying to breathe when his lungs were filled with blood and landed on AJ's shirt.

"They stayed there until this sticker was placed on there, which preserved those stains," she said.

DeToto questioned how no other traces of blood were found on AJ Armstrong.

"The size of that spatter, the surface area of the spatter is very, very small. The diameter of a mechanical pencil. That's how small they are. And so, in flight, they are drying. And so, when they're deposited on the shirt, they're not soaking into the shirt," Rossi told "20/20."

Detoto argued that the presence of blood on AJ’s shirt could have been the result of cross-contamination.

“It’s very possible the those specks of blood got under the sticker through the handling of the evidence,” Detoto said.

Jordan, however, said he was confident the jury would side with prosecutors.

"The discovery of the blood, we don't need it to convict him," he said. "If you look at the evidence, his own words in the 911 [tape], his own words in the defendant's statement hours after the murder -- frankly, it is just overwhelming."

Following a two-week trial in which AJ Armstrong broke down when he saw his parents' autopsy photos, the jury began deliberation. After ten hours of deliberating over two days, they declared a verdict on Aug. 16: guilty.

An emotionless AJ Armstrong was remanded and was automatically sentenced to life in prison. Because he was a juvenile at the time of the crime he will be eligible for parole after 40 years.

AJ Armstrong's family was furious and maintained he was innocent.

"Right now Antonio and Dawn are not at peace about what's happening to their baby boy, not at all," Kay Winston, AJ's grandmother, told "20/20" after the verdict.

AJ Armstrong is appealing his guilty verdict. A motion, by the defense, for a fourth trial was denied in October.

DeToto said that he has visited AJ Armstrong in prison since the verdict and that he was "upbeat."

Kay Winston said she told AJ’s wife to tell their son that his father was away at work.

Jordan and Trask told "20/20" that they both felt bad for the Armstrong family, including AJ Armstrong's son.

"I think that he's going to grow up without a father in his life and that's nothing that he did," Trask said.

"Being a parent myself, you wonder how Dawn and Antonio would feel," Jordan said. "They probably forgave him the moment that he killed him."