Betty answered a knock at her friend Kathryn's house. A killer was on the other side.

On a quiet evening on Labor Day 1990, Betty Jones heard a knock on the door of her friend Kathryn Crigler’s house in Starkville, Mississipi.

Kathryn, 81, was recovering from a recent leg amputation and the 65-year-old had offered to stay with her that night to adjust to her new medication.

The friends, who had met at church, were getting ready to watch a baseball game on TV when someone rapped on the front door at around 9pm.

Knock. Knock.

Without hesitation, Betty opened the door.

It was after all a small town of 25,000 people where everyone knew each other – especially Betty who had always been very involved in the community.

She wasn’t to know the danger lurking just centimetres away on the other side of the door. 

As Betty opened the door, with no word or warning, a man stabbed her. She put up a fight, desperate to save her friend who was still in her room, but the man slashed her throat, instantly killing her.

He then found Kathryn, sitting in her wheelchair, in her room. He raped her and then strangled her, leaving Kathryn for dead. He even smoked a few cigarettes and locked the front door behind him when he fled.

But Kathryn didn’t die. Although she was left with several broken bones from the brutal attack, according to WTVA, the retired schoolteacher managed to drag herself down the hallway and into the kitchen, tugging at her rotary phone cord from the floor to call for help.

"It makes me proud of her. Because she could've totally just given up right then. Most people would… That's pretty amazing," Kathryn's granddaughter, Juky Crigler Holt, told CBS’s 48 Hours. 

When police arrived at the crime scene, Starkville detective Bill Lott described it "as bad as anything Jack the Ripper did". 

Betty’s body was found in the front room, blood everywhere, the baseball game still blaring on the TV.

“It’s tragic what happened in that house. I don’t like it. I don’t care where in the world you go - you won’t find a worse crime than this," Bill said, according to WLBT. 

Kathryn was taken to hospital, where a rape kit was performed. The results of the test were sealed in a bag, forgotten and unused. DNA testing was still years away, and it would only be decades later that advancements in DNA technology would help police find their killer. 

When Kathryn was eventually told of her friend Betty’s death, she spiralled into a deep depression, blaming herself for her friend’s murder. She never returned to the home she loved and sadly, two months later died of complications from her injuries.

The years began to tick by with police coming up with no new leads. Decades passed. 

In 2017, Betty's step-grandson, Jason Jones (pictured above with Betty), decided to start his own investigation.

Haunted by the ruthless killing of his Maw Maw Betty – a ‘taboo’ subject that was never mentioned in his family - Jason started a podcast Knock Knock in the hopes of making sense of what happened and perhaps, to uncover new information.

“I will never forget that night… I remember waking up in the middle of the night sweating,” Jason says in his podcast of the night their family received that tragic call that Betty had been viciously murdered.

Jason was only 10 when Betty died, but she had left a lasting impression on the young boy.

"She was the grandmother that was really into baseball which... for a little boy, havin' a grandmother that's into baseball is pretty cool," he told 48 Hours.

In the podcast, Jason interviewed his grandmother and Kathryn’s friends and family, investigators and the tenacious detective Bill Lott.

Bill had taken over the cold case in 2004. Having fond memories of older aunts in his own childhood, Bill felt a special connection to the case, and was determined to solve it.

“I told my wife, I said, 'If I retire and I still haven't solved this case, you're gonna be rollin' me up to the police station in a wheelchair where I'm working on it... I'm not quittin','" Bill told 48 Hours.

In 2005, Bill sent the rape kit evidence to a lab, where they developed a semen-DNA profile that police could then test other people against, reported WLBT.

"The number one suspect back then that everyone felt did it, he didn’t match right off. He’s the very first person we test," Bill said.

They tested more than 60 people. Not one was a match. There was no match on the national DNA database of violent criminals either.

But in 2018, Bill heard about new cutting edge technology - DNA phenotyping, a process which uses killer's DNA profiles to sketch out what they look like

From the DNA, a lab were able to generate a mug shot of what the suspect might have looked like 28 years ago and what he might look like today, according to AP. 

"The DNA predicted that he would have light brown hair and blue eyes. [Kathryn had] lived long enough to give three interviews. She said the suspect had blonde hair and blue eyes," Bill said. 

"It's like, now I know what you look like. Now I'm going to get you." 

The suspect’s DNA also underwent genetic genealogy, where the DNA sample is compared to public databases where people have voluntarily submitted their samples.

It was genetic genealogy that sensationally led to the arrest of a suspect in the Golden State Killer in California in 2018.

And it was genetic genealogy that would eventually lead to the breakthrough of a crime that many thought would never be solved.

Bill was given a name by a genetic genealogist. A suspect. But to ensure it was the right man, he sent a cigarette butt taken from the crime scene to the lab to test on Friday.

The next day, they had their results. It was a match for the semen-based DNA profile developed more than a decade ago.

On October 6, 2018, nearly three decades after the brutal murder and rape and just two days before what would have been Betty’s birthday, the Starkville Police Department arrested and charged Michael Devaughn, 52, of Reinzi, with capital murder and sexual battery.

He was already in jail on unrelated drug charges and it was only because of his recent criminal history, Devaughn's DNA was in the database. 

BREAKING: Suspect arrested in Starkville Labor Day Murder from 1990. Michael W Devaughn, 51, is in custody. Charges pending. pic.twitter.com/l57vFxfdjC

— WCBI News (@WCBINEWS) October 7, 2018

For Jason, the arrest finally put an end to his 30 year search for answers.

He told the Clarion Ledger, it was time to "observe and find closure and healing through this process".

"This is the time for us remember her and remember the life she led, rather than remembering how she died," he said.

For Bill, it was an emotional moment as he spoke about the case he had dedicated years of his life to.

“I don’t think people realise when I was walking around here in a haze, I wasn’t talking or smiling - I like to smile. I’m normally happy. I hope I get my smile back. I’m normally a happy person,” Bill said. "It’s a long journey. Just got a little ways more to go.”

He praised the hard work of everyone involved in the investigation but would not say more for fear that “loose lips” could impede the case.

If convicted of capital murder, Devaughn faces life in prison or the death penalty. He could face up to 30 years if convicted of sexual battery, reports the New York Post.