10/20/2014 12:44 EDT | Updated 12/20/2014 05:59 EST

Derek Puffer told police he killed Bill and Donna Lee Powers

Derek Puffer of Calgary has pleaded not guilty in the killing of his mother and his stepfather, well-known sportscaster Bill Powers.

Puffer's trial began Monday in a Calgary courtroom. He formally pleaded not guilty in the deaths of Bill and Donna Lee Powers.

Puffer's lawyer says the issue in this case is not whether Puffer committed the crime, but whether he is not criminally responsible because of mental illness.

Bill Powers retired in October 2009 after almost 50 years in the sports broadcasting and communications business.

The couple were found dead in their Braeside home in July of last year. Bill Powers had been stabbed 25 times and Donna Lee 16 times.

The court heard that Puffer, 40, was found that night naked with blood on his hands at a nearby bus shelter at 14th Street and Southland Drive.

He told police he had killed his mother and step-dad, and that he suffers from schizophrenia and hadn't taken his medication for nearly 20 years. He told police that the Powers had put a computer chip in his brain and that he was hearing voices asking him about a knife.

Const. Michael Green, who found Puffer, described the accused as "stoic, emotionless and calm … cool as a cucumber."

The court saw a video recording of an interview between police and Puffer after his arrest. He told detectives that he blacked out a lot and did irrational things. "It hasn't registered that I killed them," Puffer said.

Puffer did not have a history of violence, but in an agreed statement of facts read in court, Bill Powers had expressed concern to several people about his safety around Puffer.

The trial is expected to last all week.