After Allan Shyback told an undercover officer he'd strangled his common-law wife and hid her body in the basement of the home they once shared, police used sledge hammers, picks, chisels and hammers to break up a cement structure where Lisa Mitchell's body was found inside a plastic tub.

That is some of the evidence heard at Shyback's second-degree murder trial, which began on Tuesday in Court of Queen's Bench in Calgary. He pleaded not guilty to the murder charge and one count of indignity to a body.

Cst. Karl Sudyk, who was the primary investigator from the missing persons unit, described the tomb-like structure that had been constructed in a corner of the basement. On top of the enclosure, police found an air freshener, cat litter, a bottle of bleach and a blanket.

Police found a homemade concrete tomb in Allan Shyback's basement. Using tools, officers uncovered a tupperware bin with Lisa Mitchell's body inside. (Court exhibit)

Mitchell, 31, was last seen by her mother on Oct. 29, 2012. Two years later, a lengthy undercover operation ended with the accused's confession and the discovery of Mitchell's remains in the same home where her two children continued to live with Shyback.

Sudyk examined Shyback's financial records which showed on Nov. 3, 2012, just days after Mitchell's disappearance, he purchased six bags of concrete and a pail.

Allan Shyback, centre, is accused of keeping his common-law wife's body hidden in the basement of their home for two years. (Meghan Grant/CBC)

Mitchell and Shyback had been in an on-again/off-again relationship for 10 years. Mitchell spent the weekend of Oct. 27, 2012, with her mother, Peggy Mitchell, in Longview so she could work at the local bar.

The mother testified that when her daughter left on the Sunday, it was the last time she saw her alive.

Over the next two weeks, as the mother tried to find out what happened to her daughter, she began receiving what she described as out-of-character emails from Lisa.

"Just had to get away," read one of the messages.

In another, Lisa asked her mother to pack up her clothes but she never showed up to collect her belongings.

Shyback spliced voicemail together

Finally, a strange voicemail triggered the mother to contact police.

"Hey, I'm OK. Sometimes my weeks get crazy you know and it was quiet for a while and then all of a sudden this happened and within a half a day I should but he's back now and I gotta go. Love you."

It was her daughter's voice but Mitchell said something about it wasn't right.

Lisa Michelle Mitchell's remains were 'excavated' from the basement of the home she shared with Shyback in December 2014. (Calgary Police Service)

"It made me even think more it wasn't her, it sounded like her voice but I know my daughter and I know how she talked but it wasn't her," she said. "It was totally, totally off."

Shyback told an undercover police officer that he spliced together old recordings of Lisa's voice to create a message he sent to her mother two weeks after her disappearance, according to an agreed statement of facts.

On Dec. 4, 2013 — a year after Lisa Mitchell disappeared — the Calgary Police Service embarked on Operation Aurora, an undercover investigation involving 83 scenarios where police befriended Shyback over a 12-month period.

On Dec. 5, 2014, Shyback was arrested by Calgary police officers in Winnipeg. The same day, police executed a search warrant on Shyback's home.

Forensic analysis of Shyback's laptop seized after his arrest, showed searches on how a body decomposes and murder legislation in Canada.

The IP address attached to Shyback's laptop was the same one used to send emails to Peggy Mitchell.

First sign of a defence

Medical examiner Dr. Jeffery Gofton testified that by the time he examined Mitchell, her body was in a significant state of decomposition.

He found the cause of death to be "probably manual strangulation" based on a fractured thyroid cartilage in Mitchell's neck.

But Gofton confirmed for defence lawyer Balfour Der that Mitchell's fatal injuries could have been inflicted almost instantly and her death occurring within seconds.

Gofton also confirmed to Der that Mitchell could have struggled or fought even after her thyroid cartilage fractured.

Der's line of questioning was the first sign the defence could be heading toward an argument of self defence, which would mean an acquittal, or a finding of guilt of the lesser offence of manslaughter.

Prosecutors Jayme Williams and Heather Morris will also call seven police officers to testify over the coming days.

It is unclear if Der and his co-counsel Eleanor Funk will call Shyback to testify during the trial.

Court of Queen's Bench Justice Rosemary Nation is expected to hear evidence for about a week and a half.