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?

Produced by Susan Mallie
[This story first aired on Feb. 9. It was updated on Aug.3]

(CBS) La Crosse, Wis. -- When lead investigator John Christophersen arrived at the Koula household on May 24, 2010, his mind was spinning.

"What in the heck could've happened in a situation where two people are dead? Because this is an affluent neighborhood and crime is very limited in this part of the county," he told "48 Hours" correspondent Peter Van Sant.

Standing outside the house was son Eric Koula, who had discovered the chilling death scene.

"I just didn't understand why, why they're like this at all. Just great people. You know they're (sob) now they're gone," Eric cried.

Christophersen, a special agent at the time with the Wisconsin Department of Justice, was assisting the La Crosse Sheriff's Department. He takes "48 Hours" step by step through the Koulas' four-bedroom home.

"And we're entering from the three car garage into the kitchen/mudroom area," Christophersen showed Van Sant. "I walk into the kitchen area, I notice an elderly male lying prone on the kitchen floor, face down, a pool of blood around him. He was shot the moment he walked in the door."

Dennis Koula was 68 years old. A wealthy businessman, he once owned a chain of pharmacies and a Ford dealership in town which he sold in 2006.

"Up until when we sold the dealership I worked with my dad six days a week, side by side," said Eric.

When asked to describe his relationship with his parents, Eric told Van Sant, "Oh, very close."

So close, in fact, Dennis gave Eric hundreds of thousands of dollars to start a career as a day trader, buying and selling stocks from a computer in his house. Cindy Cowell, Eric's younger sister, also received money from their dad, but it was just a pittance compared to her brother.

"My parents were very good people. They were hard workers," she said.

Cindy says her dad could have retired long ago, but he still worked part time.

"My dad often joked, 'It would be better for me to go to work at the pharmacy and get paid for it than stay home, because your mom will have me working for nothing,'" she recalled with a laugh.

Eric and Cindy, each married with families of their own, have been close since childhood; the bond with their parents was even closer.

They both lived within minutes of their mom and dad's house.

"One of the last conversations I had with my father, " Cindy said with a sniffle, "he told me that I could count on him. ... 'I'm always there for you. Love you, honey. Bye.'"

"And now we're going into the computer room where Merna is located,"

Christophersen said, continuing the tour of the crime scene. "Merna is slumped over the computer. Her left arm is still on the keypad. Her right arm is resting on her right knee."

"What is that suggestive of for you?" Van Sant asked.

"That she was not startled. She was comfortable sitting there. She died almost instantaneously and she did not know what was coming," said Christophersen.

Like her husband, Merna, 65, loved to work. She was a substitute teacher.

"She especially liked teaching junior high," Cindy said. "She was outgoing, fun-loving ... She loved Christmas because of the expression on everybody's faces when they opened presents."

"She would -- always wanna talk about ... anything goin' on in your life, talk about the kids," said Eric.

The morning of May 24, Merna was scheduled to work at nearby West Salem Middle School. When she didn't show, Eric received a call from a concerned school secretary.

"I called my mom's cell phone and there was no answer there," Eric said. "'Well, that's weird. I'll call my dad.' So I called my dad's phone. And he didn't answer."

Dennis should have been on his way to work at the pharmacy.

"'Oh, boy -- did they have a car accident?' Or, you know, 'What's goin' on?'" Eric wondered.

Eric's wife, Christine, was at home with him that morning.

"He said, 'Well, you know, it just doesn't -- it doesn't sound like them -- something's not right,'" Christine recalled. "He said, 'I'll run out there and see, you know, what's goin' on.'"

Eric hit the road. It's about a 10-minute drive to his parent's house.

"I drive up the road to the house. ... And I see my dad's truck out in front," Eric said. "And I look inside and I see my mom's Mountaineer in the garage."

Puzzled, Eric walks through the garage and enters the house.

"I opened the door and I see my dad on the floor," he said. "And I grab my dad's leg, shake him. He doesn't move, doesn't respond. Nobody's saying anything, anything back."

Hysterical, Eric dials 911:

911 operator: Ok, and your dad is not breathing?

Eric Koula: No.

911 operator: OK, alright. Is your father alive?

"I remember talking to the lady on the phone, asking various questions," said Eric.

911 operator: Is your mom there also?

Eric Koula: I, uh, MOM! (cries)

911 operator: Is your mom breathing?

Eric Koula: No (crying)

"And that's when I saw my mom in the computer room around the corner. And her head was down on the keyboard," Eric continued. "I miss them so much."

"Is this an ambush, these two killings?" Van Sant asked Christophersen.

"Possibly. It could've been. Or certainly somebody that they knew who came up behind them, lying in wait," he replied.

As Christophersen continues touring the crime scene, he's struck that so many valuables have been left untouched.

"I see electronics, I see a wallet that's undisturbed, I see a purse that is undisturbed, a computer sitting out," he said. "I see credit cards and cash in closets and in dresser drawers that are undisturbed. I see jewelry boxes that haven't been touched."

Something else catches his eye: the dresser drawers.

"An individual who's going to burglarize a place is going to open up the top drawer, shuffle through it, close the drawer and then look into the second drawer. If you look at this they're indicating that they go to the bottom drawer first, pull it out, shuffle through it then pull out the next drawer and so forth going up," Christophersen explained.

"Who does that?" Van Sant asked.

"Nobody that I know of," Christophersen replied. "And the kids, Eric and Cindy, go into the residence and they're telling us that there's nothing stolen from the residence."

"Who would have any motive to kill the Koulas?"

"At this point, we don't know. There's nobody that stands out."

Investigators would not have to speculate long. Just days after Dennis and Merna are found dead in their home, a neighbor, Steve Burgess, a president at a local bank, provides the first lead in the case.

"Steve Burgess came forward to the sheriff's department, stating that he had been receiving threats," Christophersen told Van Sant. "And they were distinctly death threats."

"Burgess is suggesting there may be a case of mistaken identity? That someone out to kill him killed the Koulas instead?" Van Sant asked.

"Yes," Christophersen replied.

But how could such a fatal mistake have been made? Easily -- if the killer used the internet.

"In fact, when you Google Earth, Steve Burgess' address ... the zoom into the house goes to the Koula's house, not to Steve Burgess' house," Van Sant pointed out.

"Yes," Christophersen said. "And now, we've got a lot of work to do because whoever committed this homicide certainly has a jump on us."

"It has all the markings of a hit, I've been told," said Eric.