Opening statements were presented today in the capital murder trial of the son of a retired NFL player accused of shooting dead both of his parents.

Prosecutors said house alarms and motion detectors indicated that 19-year-old Antonio 'AJ' Armstrong Jr killed his father, Antonio Armstrong Sr, and his mother, Dawn, in their home in Bellaire, Texas, on July 29, 2016.

AJ Armstrong claimed that there was a masked intruder inside his family's home on the night of the killings.

Trial begins: Antonio Armstrong Jr, 19, is pictured in court on Day 1 of his capital murder trial stemming from the 2016 deaths of his parents

Antonio Armstrong Sr (second right) and his wife, Dawn (second left) were shot dead as they slept; son AJ (far right) claimed there was an intruder

However, jurors were told during opening statements that no one forced their way into the home and that AJ was the only person detected walking towards his parents' second-floor bedroom just moments before the shooting.

'The killer didn’t come from outside the house,' Harris County Assistant District Attorney John Brewer said. 'The alarm tells us more than just the killer came from inside the house. The alarm tells us that the killer came from upstairs.'

Brewster added that Dawn Armstrong was shot twice in the head and died likely not knowing 'it was her son that killed her,' KTRK-TV reported.

AJ Armstrong was 16 years old at the time of the double homicide. Now aged 19, he is being tried as an adult on counts of capital murder. He has always maintained his innocence.

Defense attorney Rick DeToto countered the prosecution's statements by claiming that investigators decided early on that the teen killed his parents as they slept, then contorted the case to fit their opinion.

A prosecutor said during opening statements that house alarms and motion detectors indicated that AJ killed his parents in their home in Bellaire, Texas

Antonio Armstrong Jr, pictured listening to opening statements, would face a life sentence if convicted

DeToto also told the court his client's DNA and fingerprints were not found on the gun recovered from the scene, and that his parents' blood was not on him, reported KPRC.

When police arrived at their home at 1.40am, they found a gun in the kitchen along with a note which read: 'I have been watching you for some time.'

Police said they believed Antonio Jr, who called himself AJ, wrote the note to try to move suspicion off of him.

In his statement, the defense lawyer pointed out that AJ's older brother, Josh, lived just two minutes away from their parents' house and that he refused to provide an alibi for the night of the killings, yet he was never considered a suspect.

'Confirmation bias and reasonable doubt is weaved throughout this whole case,” DeToto said. 'They accused AJ of murdering his parents before all the evidence was in.'

DeToto argued that Josh Armstrong had a history of disturbing behavior, including multiple trips to psychiatric institutions over the past three years, and that he allegedly had access to his parents' residence.

The elder Armstrong was a standout linebacker for Texas A&M and had brief stints with the San Francisco 49ers and Miami Dolphins. He and wife Dawn owned a chain of gyms

'You're going to hear evidence that he's suicidal. You're going to hear evidence that he's homicidal. You're going to hear evidence that he's catatonic. You're going to hear evidence that he has a woman in his head that he's trying to kill,' AJ's lawyer told the court.

Antonio Armstrong Sr was a standout linebacker for Texas A&M and a first-team All-American who was taken in the sixth round of the 1995 NFL draft. He had brief stints with the San Francisco 49ers and Miami Dolphins.

AJ Armstrong has maintained his innocence from the time of his arrest

At the time of their deaths, Antonio and Dawn Armstrong, both aged 42, operated a chain of fitness centers in the area.

In the weeks before their deaths, Antonio and Dawn had caught AJ smoking marijuana and had taken away his phone and car keys as punishment, according to prosecutors.

AJ had been under-performing academically and had been taken out of private school to be enrolled in a public school, where he had failed to start on the football team.

If convicted of capital murder, AJ will face a life sentence with a possibility of parole after 40 years, reported Chron.com. The death penalty is not on the table for the defendant because he was a minor at the time of the killings.

Armstrong Jr's trial is expected to last about a month.