Inside Texas Doctor’s Plot to Kill Rival Doctor in Love Triangle

— -- Richelle Shetina thought she'd found the perfect partner in dance and in life when she started dating Joseph Sonnier.

"I loved the way that he made me feel. I absolutely loved it," Shetina told ABC News' "20/20."

But their relationship came to a tragic and abrupt end when Sonnier was found murdered in his own home.

In a case involving a love triangle and a jealous lover, police got their biggest break in solving the murder thanks to a tip from the killer's houseguest.

Read below to see who the key figures are in this story:

Dr. Joseph Sonnier

In 2012, Dr. Joseph Sonnier, 57, was a respected and wealthy pathologist living in Lubbock, Texas. He enjoyed foreign travel, fine wine and spending time with his family.

At 19, Sonnier married his high school sweetheart Becky Gallegos. Gallegos and Sonnier had two sons, Dallas Sonnier and James Sonnier.

After 27 years of marriage, Gallegos and Sonnier divorced. Gallegos remarried, and in July 2010, Gallegos was murdered by her new husband, who then killed himself.

After the divorce, Sonnier, then 48, was single for the first time since high school and received dating advice from his son Dallas.

"He went, I think, on a lark to a dance class -- ballroom and salsa and swing and things like that," Dallas Sonnier told "20/20." "And through that dance community he found his confidence again."

“I don't think he would have gotten married again,” Joseph Sonnier’s sister Missy Bartlett told “20/20.”

Sonnier began dating again and later introduced his family to Richelle Shetina. The two traveled together to Los Angeles, California, to meet Sonnier's family and to Paris, France, on vacation.

However, on July 11, 2012, Sonnier was found shot and stabbed to death at his home, almost two years to the day after the death of his ex-wife.

It was his landscapers who discovered Sonnier's lifeless body in his home. It was immediately clear to police that it wasn't a robbery gone wrong.

"We knew that we were dealing with something that was a hit, you know, of sorts … somebody had come to this house and to kill this person specifically," Lubbock police Det. Zach Johnson told "20/20."

Richelle Shetina

Richelle Shetina was a single mother when she met Sonnier at a ballroom dance class.

Like Sonnier, Shetina was divorced, and she had four sons. Shetina and Sonnier dated for about 10 months.

According to Sonnier's family, Shetina repeatedly told Sonnier how much she wanted to get married. But Shetina said there was never a discussion about marriage.

"That's not true. What I was looking for was a committed relationship with a good person. And that was it. That was absolutely it," Shetina, 53, said.

After Sonnier was found murdered, police questioned Shetina, who told them she believed someone had been watching her and following her. She also told police that she thought someone once spied on her and Sonnier.

When asked by police about anyone in her past who may have been involved, Shetina named her ex-boyfriend, Dr. Thomas Michael Dixon. After they broke up, she started seeing Sonnier.

Dr. Thomas Michael Dixon

Dr. Thomas Michael Dixon, also known as "Mike," was a plastic surgeon and had a spa in Amarillo, Texas, where Shetina went to get Botox injections.

"He was funny. He was very witty, you know, he was enjoyable. If it can be enjoyable to have somebody put needles in your face, you know, I guess that's about as enjoyable as it could be," Shetina said.

Shetina told police Dixon wanted her to "come back to him" even though she was seeing Sonnier.

Police interviewed Dixon after Sonnier's murder. "He was obsessed with Richelle. He was having a lot of problems getting over this break-up. He just couldn't seem to let her go," said Det. Johnson.

Dixon eventually moved on and started dating a medical student named Ashley Woolbert, who later became his alibi. Dixon was at dinner with Woolbert the night of the murder in Amarillo.

Authorities received a tip from a man who said his friend named David Shepard told him he killed a man in Lubbock. Police learned that Shepard was friends with Dixon. Investigators also discovered surveillance videos of Dixon and Shepard together the day after the murder.

Police later arrested Dixon for ordering the murder of Sonnier. Dixon's first trial for his role in Sonnier's death ended in a mistrial after the jury couldn't come to a verdict.

After a second trial, Dixon was found guilty of murder and sentenced to life in prison without the possibility of parole. Dixon's lawyers maintain his client never ordered a hit on Sonnier and are appealing the verdict.

Paul Reynolds

Paul Reynolds is a registered nurse. While down on his luck, his childhood friend David Shepard let him sleep on his couch.

While staying at his home, Shepard started acting strangely, told wild, violent stories and even attempted suicide, according to Reynolds. Reynolds said Shepard told him he killed a man in Lubbock.

After searching on Google and learning the murder was real, Reynolds called the Lubbock Police Department. Investigators learned that Shepard was a good friend of Dixon.

"Believe it or not, there are good people in the world, and Paul Reynolds is one of 'em," said Johnson.

Dave Shepard

After receiving Reynolds' tip, police spoke to David Shepard, who was divorced and unemployed with a string of failed business ventures.

Shepard told investigators he and Dixon bonded over cigars and martinis at local hangouts in Amarillo.

Shepard also said he and Dixon would commiserate about their failed relationships, including Dixon's dark obsession with Shetina.

Shepard explained to police that he and Dixon initially tried to tarnish Sonnier's reputation in the hope that Shetina would break up with him.

As the months went by, Shetina was still seeing Sonnier. Dixon became desperate, Shepard told police. Shepard said he then spent months following and watching Shetina and Sonnier in Lubbock in order to dig up dirt on Sonnier.

When Shepard failed to find anything about Sonnier, Shepard said Dixon eventually decided he wanted Sonnier killed.

Shepard told police he waited in Sonnier's backyard armed with a gun. When Sonnier returned home from work, he mixed a drink, police said, and shots rang out moments later.

"He's laying on the floor, not moving, not breathing, nothing. I checked his pulse in his neck. He's dead," Shepard is heard telling police about the murder in interrogation obtained by "20/20."

Shepard said Dixon paid him with three bars of silver and a box of Cuban cigars. Police found the gun, Shepard used in a lake behind Dixon's office and learned the gun was registered to Dixon's brother.

Investigators also discovered surveillance videos of Dixon and Shepard together the day after the murder.

David Shepard accepted a plea deal in 2013 for Sonnier's murder and is currently serving a life sentence.

Haley, Abigail and Rachel Shepard

Shepard's three daughters Haley, Abigail and Rachel Shepard were present when their father took the stand at Dixon's trial for murder.

"We were there more in support of the Sonnier family," Haley Shepard told "20/20." "Our heart was broken for them."

Shepard was supposed to be the keystone of the prosecution's case against Dixon, but while testifying, Shepard said Dixon had nothing to do with the murder and that he acted on his own.

Shepard's daughters said they were so disgusted and apologized to the Sonnier family after their father's testimony. They also visited their father in jail to confront him.

"Daddy, tell the truth. You didn't say anything, you just kept saying that you wouldn't tell them," Rachel Shepard is heard telling her father during their jail visit in a video obtained by "20/20."

When the jury got the case, they were unable to come to a verdict, and a mistrial was declared. At Dixon's second trial, Haley Shepard took the stand as a witness for the prosecution.

The girls said one night their father, who typically didn't have cash, treated them to dinner and bragged about buying a new grill and new tires for his car. Haley Shepard also received a new cell phone.

"And we all asked him, How do you have money?" And his words were, "I did some work for Mike, and he paid me early," Haley Shepard said.

Haley Shepard's testimony about her father was crucial at Dixon's second murder trial. The jury deliberated on the case for less than three hours and found Dixon guilty.

Andrew Dixon

Dixon's son Andrew Dixon is convinced his father is innocent.

"He was a good dad. He still is a good dad," Andrew Dixon told "20/20."

He is trying to accept that his father will likely never leave prison.

"It's unreal. It's like a nightmare," said Andrew Dixon.

Dallas Sonnier, James Sonnier, and Phillip Prestwood

Almost two years to the day their mother was murdered, Dallas Sonnier and James Sonnier said of the time, they learned their father Joseph Sonnier was murdered.

"I knew how to plan a funeral. I knew how to scrub blood off the floor. I knew everything we had to do, because I had already done it once before," Dallas Sonnier said.

"I got the call the day before the two-year anniversary of my mother's death. I thought maybe it had something to do with her death. That's how out of left field and completely out of nowhere this was," James Sonnier told "20/20."

Joseph Sonnier was also a father figure to Phillip Prestwood, who Sonnier considered a son.

"We started racking our brains. Well, it's got to be some type of girlfriend situation 'cause that's the only other people in his life out here. If it's not a robbery, it's somebody he knows," Prestwood told "20/20."

After Dixon was found guilty for his role in Joseph Sonnier's murder, the Sonnier brothers finally found closure.

"When you look at the photos of [Mike Dixon]: smug smile. When you look at his mugshot from his arrest: smug smile. And he has had that smile for both trials," Dallas Sonnier said. "And it was so great to be able to know that we finally wiped that smile off his face."