A Saskatoon judge says undercover police played by the rules set by the Supreme Court when they ran a sophisticated "Mr. Big sting" on accused killer David Caissie.

Justice Richard Danyliuk ruled Wednesday that statements Caissie made to undercover officers can be used at his first-degree murder trial at Court of Queen's Bench in Saskatoon. Caissie is charged in the August 2011 death of Carol King near Rosetown, about 110 kilometres southwest of Saskatoon.

"The admission of these statements does not mean the Crown's case will, or will not, be sufficient to meet the ultimate onus of murder beyond a reasonable doubt," Justice Danlyliuk concluded at the end of his 162-page ruling.

"It simply means the statements ought to be put before the trier of fact at the trial proper because the Crown has demonstrated either that the statement is inherently trustworthy, or that its reliability can be sufficiently tested at trial."

Police ran an undercover operation where officers posed as criminals to gain Caissie's trust, in what's commonly called a "Mr. Big sting." It was during his interactions with officers that Caissie apparently confessed to killing King and disposing of her body.

Crown and defence lawyers argued during a 'voir dire,' or trial within a trial, about whether the evidence gathered during that sting should be admissible in Caissie's trial.

Since 2014, the onus has fallen on prosecutors to convince judges to allow material from Mr. Big stings to be admitted. The Supreme Court made the change because of concerns that vulnerable suspects were being preyed upon by police.

​Prosecutor Matthew Miazga argued there was nothing in the evidence suggesting police violence or coercion of Caissie during the 49 scenarios they set up over a five-month span.

Miazga also said there was nothing that came out of the hours of police audio recordings that indicated he had been taken advantage of.

Further, he said that Caissie revealed details to officers that explained what happened on Aug. 6, 2011, the day King is believed to have been killed.

Defence lawyer Kevin Hill said the main problem with the findings in the sting is their reliability.

"There are just too many things Caissie doesn't know that he should, and things that he says he knows that don't check out," he said.

"These are details that should be seared into his memory."