Produced by Paul LaRosa and Peter Henderson
This story first aired on Jan. 29, 2011. It was updated on Aug. 6.]
On Valentine's Day weekend in 2009, Alec and Cathy McNaughton celebrated their love for each other over breakfast.
"The morning of February the 15th I got up early; I brought my wife coffee in bed. And then we had our Valentine's celebration," Alec McNaughton says. "I gave her a card. She gave me a card..."
They expressed their appreciation for each other in handwritten notes in those cards. They even exchanged chocolates.
"He painted a picture of lovers ... and talked about how sweet and caring and tender they were with each other," said Criminal Investigator Jason Fetner.
But what began as a romantic celebration ended in a most devastating way.
Investigator Jason Fetner was only 29 and had just started working homicides in Coweta County, Ga., not far from Atlanta. This was his first major murder case and it was a doozie.
"Mrs. McNaughton was wearing jewelry, gold and diamond jewelry on her person that had not been broken or ripped off or stolen," he told "48 Hours" correspondent Richard Schlesinger.
Fetner says the crime scene "looked like a horror house that you might go in on Halloween."
Cathy McNaughton had been stabbed more than 30 times. She was a most unlikely victim.
"By all accounts, Mrs. McNaughton was a really nice person with no enemies. Everybody loved her," said Fetner.
"She was definitely chatty, very chatty. We called her Chatty Cathy," said Michelle Mendenhall, Cathy's youngest daughter. "She always saw the best in people, always gave people the benefit of the doubt."
Cathy had retired after a long career at Delta Airlines - "worked from the bottom up" Michelle says - and was in training to join an investment firm.
"So your mother was really sort of a self-made woman," Schlesinger noted.
"She was," Michelle said. "She always was independent, financially independent. She used to always say, 'Don't rely on a man,' you know, financially."
But emotionally, she was not as independent as she would have liked. After nearly 21 years of marriage, Cathy got a divorce.
"I don't think she thought in her middle age she'd be divorced," said Heather Mendenhall, Cathy's eldest daughter. "It's kind of like, I'm 50 years old and, you know, I'm lonely."
Cathy's two adult daughters say their mother dated, but there was no one special. "She wanted to be happy and grow old with somebody," said Heather.
Cathy went looking for love online. That's where she met Alec McNaughton, a lawyer from Enid, Oklahoma.
"She was intelligent. She was well-read, well traveled," McNaughton told Schlesinger in an exclusive interview. "We had a lot of those things in common. I mean, love is sumthin' that just happens. There's not a whole lotta logic to it sometimes."
Alec McNaughton was 55 and had been married three times. Cathy fell for him quickly and hard.
"They had dated for about five, six months and then they were already talking engagement," Michelle said. "We definitely questioned like, 'Mom, don't you think this is a little too fast?' And she would just be like, well, 'He's it. He's it. He's my soul mate.'"
On Nov. 15, 2004, less than one year after they met, Cathy and Alec McNaughton became husband and wife.
"It was a small wedding. My mom wore a big white dress," said Heather.
"Was that out of character for your mother?" Schlesinger asked. "Did she do things quickly, impulsively..."
"No," Heather replied.
Asked what she thought of this, Heather said, "I thought that she was happy."
There were some problems. McNaughton told Cathy he was having trouble getting his Georgia law license. So he became a car salesman, but they managed.
"How was the marriage?' Schlesinger asked McNaughton.
"Very good," he replied.
"You were happy together?"
But the good times ended when McNaughton's daughter, Alexis, from a previous marriage was killed in a car accident at the age of 22.
"This girl was a show stopper," Heather said. "... Columbia University, Women's Studies major, had interned with Hillary Clinton. She was going somewhere."
Of his daughter's death, McNaughton said, "It almost killed me. I went into a depression."
Asked if it affected his marriage, he replied, "Yeah. It affected me. And -- you don't recover from sumpin' like that. My wife saved my life."
When police questioned him after the murder, McNaughton told them he and Cathy were still very much in love.
Alec McNaughton: I loved her to the-the-the depths of my heart.
"He told me that he didn't know what happened to his wife and that he wanted to find out," said Fetner.
Alec McNaughton: I want you to find the son of whoever killed my wife, OK?
Investigator Fetner: You know we will.
Police spent hours with an emotional McNaughton over the next couple of days.
Alec McNaughton: I got nothing. Ever. My life's over (cries).
"It was important to go over the details involving his daily activities so that we could exonerate him and move on to find out who did murder his wife," Fetner explained.
McNaughton told police where he had been that day. He said he went to his mother's about an hour away.
"He said he left his home sometime around 11 a.m. or 12 p.m. that day," said Fetner.
And he had left Cathy this message on their home answering machine, saying he'd arrived there: Hey, honey, it's me. I'm up at mother's ... I love you, baby. Bye.
McNaughton also told Fetner that he did not return home until 7:30 p.m., when he discovered his wife's body.
"He was very specific about the time frame that he gave to the authorities," said Fetner.
McNaughton seemed to have a solid alibi. He was cooperative and had willingly locked himself in to a timeline that investigators could now check.
"He was allowed to leave the sheriff's office and I believe he went and stayed with his mother," said Fetner.
As the victim's husband, of course there was reason to suspect McNaughton ... but there was no physical evidence against him. Jason Fetner had to figure it all out.
"I didn't see any blood on him ... there was no blood on his hands, there was no blood his clothes," he tells Schlesinger. "What you've got is no murder weapon ... you have a husband who's a former attorney that says he didn't do it ... and a woman who's been stabbed to death. It's not your traditional crime scene."
And there was one other thing to consider: Cathy's ex-husband who lives in Texas.
"It was their wedding anniversary, that day," McNaughton said. "He was in town that day. What was his motive - he's jealous."