AN investigator has revealed he could be able to finally solve a 34-year-old quadruple murder case thanks to one cop’s last-minute decision at the crime scene.

Fresh hope for justice has been rekindled decades after four teenage girls were brutally tied up and shot dead inside a Texas yogurt shop.

The dark story has been thrust back into the light by the four-part HBO documentary series The Yogurt Shop Murders, which premiered earlier this month.

The series covers the deaths of Jennifer Harbison, 17, Sarah Harbison, 15, Eliza Thomas, 17, and Amy Ayers, 13, whose bodies were found inside I Can’t Believe It’s Not Yogurt in Austin on December 6, 1991.

Three of the girls were bound with their own clothes and gagged, and they had all been shot in the head, execution-style.

Amy was the only victim who had been shot twice in the head, suggesting she may have survived the first gunshot.

The murderer had killed the girls and then lit the building on fire, burning their corpses and destroying crucial evidence at the scene.

The lack of physical evidence caused the case to turn cold, as investigators could only find traces of DNA.

Years later, police arrested four teenage boys in connection with their murders. Two of them confessed to the killings, but a court later found they were coerced.

In 2009, the two men were released from prison, and investigators were forced to search for suspects once again.

Austin Detective Dan Jackson, who was interviewed for the docuseries, made a solemn vow to the families of the victims that the investigation was far from over.

“I work on this case just about every day of my life,” he told Deadline.

FRESH HOPE

One element that’s given Jackson hope is a key piece of DNA evidence obtained from a vaginal swab taken from one of the victims’ bodies.

A mysterious sample was found, but it was so small that the host hasn’t been identified.

However, with DNA analysis technology continuing to rapidly advance, detectives hope that it could one day lead to an arrest.

“The amounts of DNA that you need are minuscule compared to what they were just a couple of years ago,” he said.

“Now we’re down to a few cells, and we can even do it with mixtures that we couldn’t do even a year ago,” he added.

“We’re cautiously optimistic about what we can do.”

In the docuseries, law enforcement recounted going against traditional advice and staying at the scene to swab the victims for DNA.

Normally, the coroner would have taken the bodies before investigators ever had a chance.

Director Margaret Brown stressed that this split-second decision is crucial to maybe one day seeing justice for the victims.

She said, “The fact that [the detectives] convinced them to stay and swab the bodies was – thank God, because there would be nothing now, if that hadn’t happened.”

DNA PROOF

The HBO docuseries also airs a never-before-seen 1999 police interview that cops had with two of the previous suspects.

They said that they were riding around in a stolen car around the San Antonio area when they picked up a newspaper and learned of the murders.

After some prodding by officers, one of the suspects suggested that he may have blocked the incident out of his mind and didn’t remember what happened.

The Yogurt Shop Murders finale will air on HBO this Sunday, August 31, at 9 pm ET and will stream on Max.