At the start it was just a bit of fun, but things became more serious when Shawn fell head over heels for his mistress. As Shawn’s marriage fell apart, the lovers now had the chance to be together properly, but Brooke didn’t want to commit and was ready to move on. Unfortunately, Shawn decided their affair wasn’t over until he said so.
Shawn, 44, owned an insurance office in Knoxville, Tennessee, and was married to Michelle Hall. The couple had been struggling with fertility problems, but they’d used a sperm donor and now Michelle was pregnant. With a baby on the way, things should have been perfect – but Michelle didn’t know that Shawn was playing away from home.
In late 2010, he had started sleeping with his employee Brooke. At 23, she was half his age and gorgeous. But Brooke wasn’t just arm candy, she was smart and great at her job too.
But by January 2011, something had changed, and Brooke picked up the phone to call Shawn’s wife Michelle. Did Brooke feel guilty about sneaking around behind her back? Or did she want to break up the marriage, so she could have Shawn to herself?
Whatever the reason, Brooke told Michelle all about their affair. She was able to tell her what colour underwear Shawn was wearing that day, she described the pictures hanging in her family home, and even mentioned the scar on Shawn’s private parts.
When Michelle confronted her husband, he was full of excuses. He said Brooke must have seen his underwear as he was coming out of the toilet at work. He claimed Brooke had been round to the house one day to swap cars, and that she knew about his scar because they were such good friends. Michelle saw through his lies and they had a huge argument. The next day their marriage was over.
Shawn was reeling. He was angry at Brooke for exposing their affair, and even more annoyed when he discovered she didn’t want to be with him any more.
At first, he bombarded her with messages day and night. Brooke confided in friends that she was frightened by his relentless attention – and it stepped up when she made it clear they were over.
Weeks after the phone call, Shawn broke into Brooke’s home and hid in her shower until she returned. Neighbours heard her ‘bloodcurdling’ screams as he tried to assault her before breaking a window to escape. The cracked glass and broken frame cost £210 to repair and Brooke’s landlady made him pay for it.
Brooke was so scared that she took out a protection order to keep Shawn away. She told police he had also shaken and choked her in his truck before breaking his windscreen by throwing his phone at it.
The relationship was beyond repair.
Then, on 15 October, Brooke’s fully-clothed body was found dumped at the side of a road just outside Knoxville. She’d been shot three times – the last bullet was execution style in the back of her head.
The brutal killing sent shockwaves across the small community.
Straight away, the police looked to Shawn because of the protection order and he became a person of interest. A bartender said he saw Shawn and Brooke together the day she was killed.
"There was tension," he told officers.
Had Brooke given in and agreed to meet him? Did she tell him it was over for good?
A search revealed bullets in Shawn’s home and there were text messages on Brooke’s phone from him. When questioned, Shawn admitted he’d seen Brooke that day, but had dropped her off alive at a shopping centre. Police didn’t believe him. He was arrested and charged with her murder.
It started almost five years of excruciating delays for Brooke’s heartbroken loved ones. Shawn’s parents paid the £190,000 bail to get him out of prison, and Shawn swapped legal teams several times. He was even caught drink driving when he was out on bail.
The trial was delayed 22 times.
At one pre-trial, Brooke’s landlady Carol Meredith described the incident when Shawn broke into the apartment.
"She told me that he beat her head against the floor and tried to rape her," she testified.
Then she read out Shawn’s apology letter, which admitted to Carol and her husband that he’d caused the damage.
Shawn had written: "Mr Clayton, my apologies. Women tend to make us do crazy things.
Unfortunately, that night was one of them. Thank you for allowing me to make amends on this."
The evidence seemed damning, but the official trial still hadn’t started. All the delays were causing Brooke’s mother, Tina Gregg, a great deal of distress.
"It angers me," she told local press. "I am not going to stop fighting until she gets her justice."
Finally, this summer, the trial began. Shawn denied murder. The prosecution said Shawn had shot Brooke and dumped her body on the road, because he was a jealous and possessive man. He was angry that Brooke had exposed their affair, and furious that she continued to push him away.
A woman called Amy Dunglinger, who used to work for Shawn, took the stand and described how he came into work the day after Brooke’s murder "shaking and sweating". She’d heard that Brooke was dead and Shawn was a suspect.
"I said [to him] were you directly or indirectly involved? And he said 'both'," she testified.
Friends said that Brooke was frightened of Shawn’s erratic behaviour. Others said he blamed Brooke for the breakdown of his marriage.
Shawn didn’t take the stand to give his side of the story. Instead, his defence reminded the jury that there was no physical evidence to put him by the roadside.
But the motive was enough for the jury. They took just four hours to find Shawn, now 44, guilty of first-degree murder. He was sentenced to life in prison without the chance of parole.
Brooke’s mum was pleased with the verdict, but couldn’t find peace.
"There will never be closure. What’s closure? There will never be closure for me. Brooke will never be back. She’s never there at Christmas. She’s never there for Mother’s Day," she said.
After facing five years of delays, Tina vows to work with a charity to push for legislation that would help other victims’ families.
For now, Shawn continues to protest his innocence. Just what triggered him to gun down his mistress in cold blood remains a mystery.