"Well, the day started off like any other day for my wife and I. It was a happy day and it was a Sunday," Harold recalled.
When they arrived at their daughter's house, the door was locked.
"And I rang the doorbell and knocked on the glass and couldn't raise anybody," Harold said. "I thought, 'This is strange.'"
Mike's mom, Carol Sisco, was also invited that day.
"The front door was locked, which was unusual," she told "48 Hours Mystery" correspondent Harold Dow. "And so we went around to the back, and the back door was open just a little bit. And so, we walked in - couldn't find anyone."
The house was quiet and Mike and Karen were nowhere to be found.
"So, anyway, Harold went downstairs, which was Karen's bedroom," Carol explained.
"As I got down there, just as I got to the foot of the stairs, I could see Karen," Harold said. "She was layin' face down on the bed, head kinda off the right side of the bed and her arm was off there… And I said, 'Oh, Christ.' I knew she was dead. I reached over and touched her, but she was cold. And then I found Mike on the outside of the bed. And - I could see that he'd been shot a number of times and had lost a lot of blood."
"Well, it didn't seem real, you know. It just seemed like a dream," Carol continued. "But, Harold wouldn't let Betty or I go downstairs, and I'm very glad he didn't. I've got my own horrible scenes of it anyway. And then, immediately, we called 911."
It was eight years ago when Topeka Detective Richard Volle got the case that would haunt him to this day.
"I got a radio call just after two o'clock on July 7, 2002. It was a report of two dead bodies in a basement," he explained. "Well, the first thought is, you gotta consider a robbery."
Mike and Karen had been dating for about four years. On the night of the murders, they had gone to a casino north of Topeka and had won big.
"The electronics were all there, there was no forced entry. There was just nothing to suggest a robbery," Det. Volle explained of Karen's home. "In fact, we ended up finding a fairly substantial amount of cash in the pockets of the victim."
With robbery ruled out, Det. Volle focused on the victims.
"They were such a sweet couple," Carol said. "He really loved Karen. And I know Karen loved Mike. She took up fishing and they went on camping trips."
It's a mystery - who would want to kill Karen and her boyfriend?
Det. Volle retraced the couple's evening, starting at the casino.
"Looked like they were having a good time. Didn't have any issues with anyone. And in a - you know - casinos have cameras everywhere. And we didn't see any indication that they'd had a struggle with anybody, any kind of altercation." And, he said, no one suspicious was following them out.
Karen Harkness, 53, was divorced with two grown children; Chad and Erin.
"I remember telling her on several occasions that if I could be half the mom that she was to me, then I would be the best mom in the world," said Erin Sutton.
"It was a surprise to everybody that this would happen to somebody like her," said Chad Harkness.
Detective Volle was unsentimental in his investigation and looked at Karen's son, Chad, and Karen's ex-husband as possible suspects.
"I was asked to come down to the police station… I was hooked up to a polygraph machine and asked if I had murdered my mother or in any way participated in the murder of my mother," Chad explained to Dow. "They looked at my father as well."
Both Chad and his father were eliminated as suspects. But if Karen wasn't the intended target, then maybe it was Mike. His son, Dustin, couldn't believe anyone would want to kill his father.
"I mean everyone that I saw that knew him, you know, would, you know, shake his hand and give him a big smile," Dustin said.
Mike Sisco, 47, sold welding equipment in Topeka. The divorced father had custody of his two teenage children.
"He was a wonderful son, and he was a very good father," said Carol.
But Mike was having real problems with his daughter, Hailey.
"I wasn't happy with my dad. I wasn't happy with my mom. I wasn't happy at my school," Hailey Sisco said.
In the months before the murder, the 17-year-old was angry at everything, but it was her father who she really turned against.
"I hated my dad," Hailey said. "I flat-out hated him."
Things only got worse when Hailey spent more and more time with her boyfriend, Chris Seel.
"My dad hated him," she told Dow. "Chris was the bad - you know, bad boy - definitely didn't follow the straight and narrow.
"You ultimately moved out of the house?" Dow asked.
"My dad gave me an ultimatum: 'Either you follow my rules or you move out.' And I moved in with Chris. And then my dad was killed and…"
Because of that tension with her father, Hailey and her boyfriend attracted police attention. Hailey said "they searched Chris's house completely."
Mike's sister, Cathy Boots, and her husband, Mark, thought maybe the young man and Hailey could be involved in the murders.
"She was acting so irrationally, no one knew what was going on," said Cathy.
So at the same time Hailey had to deal with the murder of her father, she also had to defend herself and her boyfriend from police suspicions.
"Me and Chris were interviewed, I mean, they came and talked to us. I'm sure we were on their list of who, you know, of people who did this," she said.
"All the checking that we've done on Hailey and her boyfriend, there was nothing there that - that suggested that she could be responsible for this," Volle said.
All the leads in Topeka went nowhere.
"If money's a motive, that's easy to track. If revenge is a motive, that's a little bit easier to track. But there was nothing that was in Mike and Karen's lives that would lead to revenge," Volle said. "I mean, they didn't do anything to anybody. We just don't have any ideas."
But Mike Sisco knew who might want him dead. And he told his brother-in-law, Mark Boots, a week before the murders.
"He said, 'Mark - you're gonna wake up some morning and you're gonna find out I'm dead. And you're gonna know who did it.'"