A.J. Armstrong Re-Trial: The problem with DNA evidence

HOUSTON - Often DNA evidence helps prove a prosecutor's case but not in the Capital Murder trial for Antonio Armstrong Jr. 

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A DNA expert just left the witness stand, and although she testified for the prosecution, what she told jurors did not prove who murdered former NFL player Antonio Armstrong Sr. and his wife Dawn Armstrong.

Jurors often want to know a killer's DNA was found at a crime scene. However, in the Capital Murder trial for Antonio Armstrong Jr, who’s charged with murdering his parents as they slept, DNA Expert Courtney Head says this case isn’t that simple.

Head says because kids and parents share DNA it’s hard to pinpoint or exclude any Armstrong family member (parent or child) as the one who left touch DNA on the gun, on the pen used to write the "I’ve been watching" note left beside the gun and the kitchen drawer where the pen and paper were retrieved. 

RELATED: Jurors see activity on cell phone the morning his parents were murdered

Antonio Sr. and Dawn Armstrong were shot to death in their Bellaire area home in 2016 when Antonio Junior was just 16 years old.

A firearms expert also took the witness stand saying the murder weapon was a semi-automatic, not automatic, meaning the killer had to decide to pull the trigger to shoot Mrs. Armstrong twice and Antonio Sr. once. Both were shot in the head.

MORE: Officers describe Armstrong's demeanor after murdering parents

Prosecutors say Antonio Jr. killed his parents as they became increasingly upset with him for failing grades, alcohol and drug use.

Defense attorneys point to flirty text messages between Antonio Jr. and his girlfriend an hour and a half before his parents were murdered. 

"Within 90 minutes of him supposedly executing his parents he’s having kissy faces with his girlfriend," says Defense Attorney Rick Detoto. "That makes no sense whatsoever. I've always thought the text messages help us way more than they harm us."

CONTINUOUS COVERAGE OF AJ ARMSTRONG TRIAL

A home alarm expert is expected to testify Wednesday that the alarm was set and no windows or doors were opened for hours before the Armstrongs were murdered.