How 34-year-old Yogurt Shop Murders of four teen girls who were tied up, stacked on top of each other and shot in the head before being burned is now subject of documentary trying to unravel mystery

Four teenage girls were shot dead execution-style at a frozen yogurt shop more than three decades ago in a chilling cold case that continues to haunt their Texas community today.

The bodies of Eliza Thomas, Jennifer Harbinson, Sarah Harbinson, and Amy Ayers were found charred beyond recognition inside the storage room of the I Can't Believe It's Yogurt shop in Austin on December 6, 1991.

The girls were found naked, gagged, tied up, and stacked on top of one another, each having been shot in the back of the head. At least one of the teens had been raped.

Investigators believe that after they were attacked and killed, their murderers started a fire using lighter fluid and paper products from the shop to cover up their crime.

Detectives first centered investigative their investigation around four teenage boys: Robert Springsteen, Michael Scott, Maurice Pierce, and Forrest Welborn. Scott and Springsteen confessed to the killings while in police custody. 

Scott was sentenced to death and Springsteen to life behind bars. But their convictions were ultimately struck down by the Texas Court of Appeals and the pair were freed.

Police have eyed more than 1,200 possible suspects over the last 34 years, including members of a Mexican motorcycle gang, and secured dozens of confessions - but the case still remains unsolved.

Dozens of people have confessed to the yogurt shop murders over the years, but the case still remains unsolved. 

The case is now being revisited in a new docuseries that premiered at the South by Southwest Festival on Monday night. It will later be released on HBO Max.

Eliza and Jennifer, both 17, were working at the I Can't Believe It's Yogurt shop in Austin on the night of their murder. 

Jennifer's 15-year-old sister Sarah and her best friend Amy, 13, stopped by with hopes of catching a ride to a slumber party after the store closed at 11pm. 

Detectives theorized the girls were forced into the storage room by at least two men, forced to undress and then bound by their undergarments.

Some of them were sexually assaulted before the attackers shot them in the back of their heads using what police believe were a .22 revolver and a .380 semi-automatic pistol.

The criminals then set fire to the shop, destroying much of the evidence and soot-covered fingerprints. Despite the damage, forensics investigators did manage to find a partial DNA profile from the scene.

But the sample only had 16 markers, which KEYE-TV reports is not enough to incriminate a suspect. It can, however, eliminate a potential suspect.

Investigators found no sign of forced entry, but noted the back door had been open - likely how the killers fled the scene. 

During the early stages of their investigation, police tracked down all of the customers who visited the yogurt shop on the day of the killings to see if they noticed anything suspicious. 

One couple told police that the girls locked up the store behind them an hour before first responders were called to respond to the fire, a report in the Austin-American Statesman claimed at the time.

Several customers recalled seeing two men who 'looked out of place' enter the restaurant just before closing, but oddly the pair did not order frozen yogurt - only beverages.

'They never have been identified,' Austin police detective John Jones told CBS News. 'And we did everything… We even hypnotized some folks.'

The case went dead for nearly eight years until 1999, when detectives opted to re-examine their old lead into Springsteen, Scott, Pierce and Welborn.

Investigators first targeted the teens in 1991 after Pierce, then a teenager, was found in possession of .22-caliber pistol at the local mall. 

He was questioned by police and admitted the gun had been used in the quadruple homicide, but claimed that it was in Welborn's possession the night of the murder.

Pierce, along with the three other boys, was dismissed after detectives ruled his confession was not credible.

The four, who by this point were of legal age, were arrested in 1999 in connection to the yogurt shop murders.

Charges against Welborn and Pierce were dropped, but police claimed that Scott and Springsteen confessed to killing Eliza, Jennifer, Sarah and Amy during their interrogations.

The pair were ultimately convicted of murder and Scott was placed on death row in 2001. Springsteen hit with a life sentence the following year. 

But the Texas Court of Appeals overturned their convictions after ruling that Scott and Springsteen had not been given the opportunity to cross-examine one another.

It also emerged that an officer held a gun to Scott's head during his multi-day interrogation, Texas Monthly reported in 2001. 

Another detective admitted to knowing that a federal ballistics report ruled out Pierce's pistol as the murder weapon months before officers arrested the four men.

Police searched the Colorado River for another gun that had supposedly been dumped after the murder, but never found it.

Scott and Springsteen were released on bond in June 2009 after new and improved DNA results did not link them to the crime scene and instead revealed the presence of an unknown male. Their charges have been dismissed. 

Two Mexican nationals were also arrested in connection to the slayings, but later ruled out as suspects in the case.

Porfirio Villa Saavedra, known as 'the Terminator', and Alberto Jimenez Cortez were taken into custody by Mexican authorities in October 1992.

Saavedra, a known member of the Mierdas Punks motorcycle gang, matched the description of a man who was seen in a vehicle outside the yogurt shop on the night of the murder.

The Mexican attorney general's office claimed the men confessed to the killings. 

However, Saavedra recanted his confession two days later, claiming that Mexican police had tortured him into confessing.

Austin police sent officers to Mexico City several times over the course of a year to question Saavedra and Cortez before eventually concluding they were not involved in the slayings.

The case remains unsolved almost 34 years later, but has recently been revisited in a new true crime series titled The Yogurt Shop Murders.

The first episode of the program, which was produced by independent entertainment company A24, debuted Monday night at the South by Southwest Film & TV Festival.

The three-part program dives into the twists and turns of the investigation into the slayings, as well as the trauma left behind by the unsolved case.

'It was a really hard story to tell, because we wanted to tell the details of the crime, but we also wanted to get what was interesting to us, which was about something deeper,' director Margaret Brown said after the screening, My San Antonio reports.

The docuseries features interviews with family members, investigators, and suspects, as well as archival footage from the decades-long investigation.

The mini series is set to air on HBO Max later this year.