LA CROSSE, Wis. -- -

It was a murder trial that shocked the La Crosse community.

Could a son be guilty of gunning down his own parents for money?

Last summer the jury said yes, convicting West Salem man Eric Koula of two counts of first-degree intentional homicide.

Now, Koula is speaking in his first sit-down interview on CBS' "48 Hours Mystery."

The prosecution painted Koula as a man driven by his financial struggles to murder his parents, Dennis and Merna Koula.

Throughout the entire process, Koula has maintained his innocence -- a side of the story he fights to explain in a one-hour CBS "48 Hours Mystery" Saturday.

The verdict was read on June 27, 2012.

"I just couldn't believe it. I don't know how they arrived at that. You know, hear, 'Guilty, guilty, guilty,' over and over and over from each of them," Koula said on "48 Hours Mystery."

Koula was convicted of killing his parents in their Barre Mills home in 2010.

Koula's sister, Cindy Cowell, said it's a reality that's hard to come to grips with.

"My heart just stopped for a second there. It's like, 'Wow!' Eight million thoughts through your head. 'Mom and Dad are dead. Their killer's going to prison; was found guilty. Wait a second -- that killer's my brother,'" said Cowell.

"48 Hours Mystery" traces the evidence and testimony that led the jury to convict Koula and asks Koula and his wife directly about a note he planted in his own mailbox to throw off the investigators.

"Four days after the Koula's bodies' were discovered, Eric Koula finds a note in his mailbox," said John Christophersen with "48 Hours."

"He opened it up. He saw the letters, and he was very upset," said Koula's wife, Christine Koula.

"What did the letter say?" asked Peter Van Sant, with "48 Hours."

"'Fixed you,'" said Christine Koula.

"I wrote the note, and I put it in my mailbox. I just wanted them to leave me and my family alone and get who killed my mom and dad. I know what I did was really stupid, and I'm sorry I did it," Koula said on the stand during the trial.

To add to the prosecution's case, Koula also cashed a check from his father on which he forged his father's signature.

"Isn't it true you only came forward with the admission that you had signed that check after a handwriting expert had come to that conclusion that it wasn't your father's original signature, and that that handwriting expert, in fact, claimed that it had been written in a very slow, hesitating way, which wouldn't be the case if you had signed your father's name dozens and dozens of times as you claim?" asked Van Sant.

"I signed his signature.  I don't believe I wrote it hesitantly and meticulously like she described.  I've signed it lots of times.  You know, I wrote it on our table, and that's not a smooth surface," said Koula.

It was cashed the day after their bodies were found

"But you hid an important fact from this investigation, and you know that looks terrible?" said Van Sant.

"Yeah, it does look terrible." said Koula.

But despite the lies and evidence piled up against him, Koula has refused to back down.