Who killed Dennis and Merna Koula?

Did a hired gun target the wrong victims or was La Crosse, Wis., couple murdered by someone they knew?

Eric Koula and Cindy Cowell had been turned into orphans in the same instant.

"It's been said that you and your father were best friends. Is that true?" Peter Van Sant asked Eric.

"Yes, we were. I still can't believe they're gone," he replied in tears.

It's difficult to say which sibling took their deaths harder.

"Cindy was completely distraught. She was emotionally wrecked," said John Christophersen.

"For my parents to die the way they did is just it's the most bizarre twist of fate ... for two people who were so against violence to be brutally shot, it just boggles my mind," said Cindy.

Two people with no known enemies had been assassinated in their home.

"Where does this investigation begin?" Van Sant asked Christophersen.

"With family and friends," he replied. "We need to formulate a timeline to figure out exactly when Dennis and Merna died so we can put together the pieces of the puzzle."

A crucial piece of that puzzle came from Merna. When she was shot, her left hand came to rest on the computer keyboard.

"We had a forensic computer expert take a look at the computer that Merna was on, and her last keystroke on that computer was at 5:41pm on May 21st," said Christophersen.

Friday, May 21 -- three days before Eric discovered his parent's bodies. The cops kept that time of death a secret.

"So no matter what timeline individuals provide to us at this point for their alibi, we now have a precise time that we can relate to as to whether or not they could've committed the crime," said Christophersen.

Investigators pieced together Dennis and Merna's last day of life. Merna was doing research on the home computer. Dennis had left the pharmacy around 5 p.m. It takes about an hour to drive home.

"I believe she died first, and then Dennis," Christophersen said. "He was shot as he was coming in the door."

Christophersen says they were both shot once with a .22 caliber rifle. Investigators began chasing their first intriguing lead -- the death threats to that banker who lived up the road. They found the culprit who made the calls, but hit a dead end.

"The individual that we believe was responsible for those threats would not have been able to carry out the homicides, because they had a legitimate alibi at the time that Dennis and Merna were killed," he explained.

So deputies began looking at family members. As it happened, the day Eric's parents were murdered was his 16th wedding anniversary. That afternoon, Eric was helping his friend, Mike Genz, do some home improvement.

"We were doing a tile surround on a shower, which, you know, the walls," said Genz.

Asked how late he worked that day, Eric told Van Sant, "I left there about 5:30."

"When he left the job that day, where did he tell you he was going?" Van Sant asked Genz.

"He told me he had to go get a plant for his wife for their -- anniversary and then they were going to eat," he replied.

"When he came back from Mike's, then he had the hanging flower plant. He said, 'Well, I'll go jump in the shower. And then we can get going. Cause it's a little bit later,'" said Christine.

Christine and Eric headed to dinner and later celebrated with friends. Investigators also questioned Eric's son, Dexter, who was just 16 when his grandparents were killed.

"Was it scary?" Van Sant asked Dexter.

"A little bit, yeah," he replied. "It started getting-- a little heavy towards the end, where I was afraid I was gonna break down in front of them and I -- I don't know. I - it -- it was difficult."

Deputies then turned to Eric's sister, Cindy, who had an alibi for that day.

"My timestamp at work proved that I got done at 4:45," Cindy said. "I ran some errands and was videotaped on surveillance cameras at local stores, purchasing items. ... and arrived home with the only vehicle we had."

"Were you able to prove you were at home?" Van Sant asked Cindy's husband, Patrick Cowell.

"Yeah. They took the Xbox machine and timelined that I was on the video game machine at certain times," he said.

Then, while the family is in mourning, the case takes a shocking turn.

"Four days after the Koulas bodies are discovered, Eric Koula finds a note in his mailbox," said Christophersen.

"I went out to get the mail. Opened the mailbox, brought the note back in," said Eric.

"And he opened it up. And he saw the letters. And he was very upset," Christine told Van Sant.

"What did the letter say?"

"Fixed you," she replied.

"Fixed you."

"Yup..."

"And how did Eric react to that?"

"He was very upset," said Christine, who calls 911 right away.

"A deputy goes to Eric's residence to -- get the note from him. And Eric is distraught. He's stumbling, he's -- obviously upset," said Christophersen.

"'Fixed you' suggests that there's a killer out there who killed his parents to somehow get back at him? Is that the notion?" Van Sant asked the investigator.

"Yes. And also, is this just the start of the family being killed? The parents are now gone. Now are they looking at Eric?" said Christophersen.

"You're thinking, 'Could we be next?" Van Sant asked Eric.

"Yeah," he replied.

The investigation into Dennis and Merna's deaths takes on new urgency. Following protocol, deputies examine the bank records of all the family members. That's when they find something very strange in Eric's account.

"On Saturday, after his parents were killed, he had deposited a $50,000 check into his account," said Christophersen.

"$50,000?" said Van Sant.

"$50,000," confirmed Christophersen.

"And who had signed this check?"

"Dennis Koula," Christophersen replied.

This was the first investigators knew of any money Dennis had given his son.

"And I know it looked terrible. I had gotten this check from my dad several days ago. I went and cashed it on Saturday. And now they're dead. And my dad, you know, was the one guy that can back me up on this. And he's not here to do that. You know, I knew I'd have to eventually explain the check and talk about it. And I could. And I would," Eric told Van Sant.

Investigators wondered, if Eric failed to tell them about the check, what else wasn't he telling them? That's when they took a closer look at his entire story -- starting with that panic stricken 911 call.

"It doesn't stand out at first, but once you take the time to listen to the 911 call, you realize that Eric never asks for help for his parents," said Christophersen.

911 Operator: What's the address to the...?

Eric Koula: I was called this morning because my mom didn't come to work and they haven't been answering their phones this weekend so I came out to check on 'em.

"Normally it's, 'Oh, my God, get somebody over here right away. Help me, help me.' And he doesn't do that?" Van Sant asked.

"Yes. 'Why isn't anybody here yet? What can I do to help my parents?' And Eric did not display that at all during the call," said Christophersen.

But the biggest break was about to come.

"We take a closer look at his financial situation, and we discover Eric Koula is broke," Christophersen said. "He is flat broke.