Mean Girl Murders: How a teen lesbian love affair ended in the gruesome killing of a 15-year-old girl's 48-year-old mom who was ambushed in her own bed and stabbed 73 TIMES

An upcoming true crime series is set to unravel the twisted case of a teen lesbian love affair that ended in a brutal murder.

Mary Ann Murphy's lifeless body was found with 73 stab wounds inside her home in the suburban neighborhood of Humble, Texas, in 2012.

The 48-year-old had been in bed when she was ambushed by her assailant.

It would later transpire that her 15-year-old daughter, Keri, and her 19-year-old girlfriend, Rebecca Keller, were behind the killing after being forbidden from seeing each other. 

Here, FEMAIL has laid bare the chilling details of the case as Investigation Discovery gets set to release Mean Girl Murders.

Mary Ann Murphy's lifeless body was found with 73 stab wounds inside her home in the suburban neighborhood of Humble, Texas, in 2012

It would later transpire that her 15-year-old daughter, Keri (pictured), and 19-year-old girlfriend Rebecca Keller were behind the killing after being forbidden from seeing each other

The gruesome tale began back in the fall of 2010 when a new batch of freshmen girls joined the Junior Varsity Choir at Chester W. Nimitz Senior High School.

Olivia Smith, who was part of the intake, said: 'Choir was pretty competitive. You want to sing and showcase what you have and sometimes that opportunity wasn't always given so you have to definitely fight for it.'

She explained that 'choir brought in so many different types of people' and among them was Keri Ann Murphy.

Friends in the docuseries described her as the leader of the pack and added that people 'gravitated towards her' because she was 'very loyal, outspoken and outgoing.'

But many admitted that she could have a 'mean girl streak' stemming from insecurity over her appearance as well as having a turbulent home life.

Keri, who had an older brother called Scott, lived with mom Mary Ann and father Don who had a 'rocky relationship' and were constantly arguing.

Olivia admitted: 'From the outside, Keri's family looked great. There was a lot of love between Keri and her mother, but Keri's parents dynamic from what I heard wasn't the best.'

This meant that Keri often appeared to be 'possessive' and fellow choir girl Antoinette Allen dished: 'There were just days that she would come to school and she would just have it out for everyone. She could get very snarky and very rude sometimes.'

Among Keri's admirers was classmate Zein Ahmed, who was described as a loner, with her friends believing he had a crush - or even an 'obsession'

Nonetheless, among her admirers was classmate Zein Ahmed who was described as a loner.

Keri's friends believed that Zein had a crush on - or even an 'obsession' with - her but that this was never reciprocated.

But things began to take a dark turn in December 2010 when the choir was practicing for their annual holiday performance.

It was during rehearsal that Keri first met Rebecca, who was in Varsity Choir, who was nearly four years her senior. 

Reflecting on the event, choir performer Amber Gresset said: 'If I could go back in time and change something, I would probably change Keri meeting Rebecca.'

Rebecca, who everyone called Bunny, was bisexual and described as 'quiet and a loner,' but she instantly had a connection with Keri.

'It was almost instant that they really liked each other. I think there was something magnetic there,' Olivia said.

Following that first meeting, their friendship developed into a relationship in just a matter of weeks.

Keri's friend Heather Tucker said: 'I never knew Keri was gay. I always thought she would follow what her mother wanted for her, but we all welcomed Keri with open arms. 

'We weren't worried about her sexuality but her parents were another story definitely - because they are southern, they are Christians.'

Rebecca had urged Keri to come out to her family just a few months into their relationship, which left her 'extremely nervous, terrified and unsure of how her mother would respond,' Olivia said.

Eventually Keri plucked up the courage and told her mom she was in a lesbian relationship.

Mary Ann ended up giving her daughter her blessing and Keri's relationship with Rebecca continued going strong.

Antoinette dished: 'They were very lovey dovey with each other - holding hands, hugging, kissing each other all of the time. 

'It didn't bother me at all but Keri was a freshman and Rebecca was a senior so the age gap was definitely pretty huge.'

Zein initially confessed but later retracted his statement with police dismissing the charges and letting him go

It was not until the spring of 2011 that Mary Ann found out about Rebecca's age and was furious that a 19-year-old was dating her 15-year-old daughter.

She told Keri to end the relationship, but the teen began sneaking out the house and lying to her mom to meet up with her older lover. 

Meanwhile, friends described how Keri and Rebecca's relationship was becoming increasingly 'unhealthy and possessive.'

Tensions increased when Mary Ann discovered that Keri had been breaking her rules and Heather said: 'The absolute final straw was when Keri's father Don came home and found Keri and Bunny in the shower together.'

The concerned mom threatened to call the police if she ever caught Keri with Rebecca again.

But, just two months later, Mary Ann had woken up in the middle of the night to check that Keri was still in her room.

She was incensed to find Rebecca in bed with her young daughter and immediately called the police.

Rebecca was arrested for trespassing and Mary Ann got a restraining order issued that forbade contact between the two girls.

But things took a tragic turn in July 2012 when Keri and Mary Ann had been alone in the family home - with her father and brother both working night shifts.

Rebecca Keller, who was then 19, pled guilty over Mary Ann's death and was sentenced to 60 years behind bars

At 1.06am Keri made a frantic 911 call from a neighbors house claiming she had woken up, heard someone busting in the back door and listened to her mom screaming at the assailants to stop.

First responders arrived and made their way through the one-story home before making a gruesome discovery in the main bedroom.

Mary Ann was found lying on her back on her bed, covered in blood with 73 stab wounds across her arms, chest, face and legs with her throat also cut.

Investigators noticed a broken window by the back door but grew suspicious since it was not big enough for an assailant to fit through and also did not allow for anyone to reach the handle of the back door.

For the most part the inside of the house did not look ransacked and officers also found a .22 caliber pistol under Mary Ann's pillow that she kept close by for protection.

The fact she had not used it seemed to suggest that she had been ambushed and was not aware of what was happening. 

Officers started to believe that it was an inside job, but with the alibis checking out for Keri's father and brother, the teen herself was asked to sit a polygraph test.

Police informed her that she did not perform well during the assessment, which was when she broke down and appeared to tell cops who killed her mom - claiming that Zein had been responsible.

She said that a few nights before the murder MaryAnn had caught Keri sneaking back into the house, but did not believe her story when she said that she had gone to investigate after seeing two men outside. 

Keri alleged that she had asked Zein to come round to the house and bang on the walls to convince her mom otherwise - but claimed things started to go awry when he turned up with a knife.

The teen said that she went across to her Mary Ann's room after hearing her scream and saw Zein stabbing her mom.

Zein himself initially denied involvement but then confessed, adding that he did not mean to kill her but had just intended to scare Mary Ann.

He told police he went upstairs, held a knife over her neck and said 'boo' before stabbing her four times in a panic.

He told police that they could find a red-handled knife in his room, which they then recovered and sent it for testing.

But his story did not stand up to scrutiny for very long.

Zein had described how he had gone 'upstairs,' but the Murphys lived in a one-story house - with Mary Ann suffering many more than just four wounds.

The knife also came back clean from the crime lab and Zein's own parents insisted that he had been home all evening - with timed and dated photos to prove it.

Investigation Discovery's Mean Girl Murders premieres Monday, March 25 

He eventually retracted his confession with police dismissing the charges and letting him go.

As a result, officers were forced to turn their attentions back to Keri who had already given two false accounts.

Cops were able to get hold of phone records from Keri's secret burner phone that she had been using to speak to Rebecca after Mary Ann had confiscated her main cell.  

They found that the duo had been talking constantly up until the day of the murder and Rebecca was brought in for questioning.

She appeared to crack under interrogation and told police that Keri had been taking about killing Mary Ann for months.

The teen lovers had reportedly waited for Mary Ann to fall asleep before Keri invited Rebecca over to the house.

Rebecca alleged that Keri had entered her mother's bedroom and stabbed her to death as she watched on from the hallway.

She said that together they had then staged the scene to look like a home invasion before also getting rid of the murder weapon and Keri's blood-soaked clothes - before planning to pin the crime on Zein over his unrequited love. 

'I would have never thought Keri was capable of murder,' Olivia said of hearing about the apparent confession - with her other friends equally unconvinced about her guilt.

Keri's grandmother stumped up the bail to get her out of jail, but the teen continued to ring Rebecca who was still locked up behind bars.

The pair appeared to be unaware that police would be monitoring their calls and, in one particular exchange, Rebecca told Keri: 'I'm glad you didn't see what happened in there.'

In response, Keri admitted that she did end up seeing the gruesome killing because, although she was in the hallway, she had peered in.

'You weren't supposed to see. I told you to stay in your room,' Rebecca had responded.

Keri and Rebecca ended up both pleading guilty. Keri was sentenced to 30 years in prison and Rebecca to 60 years.

Olivia concluded: 'If I could go back to the day that Keri met Rebecca, I would say turn away, don't do this, just listen to your mom.'

Investigation Discovery's Mean Girl Murders premieres Monday, March 25.