An undercover police officer, the prosecution's key witness, took the stand in Hamilton on Wednesday.

Crown prosecutor Fraser Kelly says the officer befriended the three accused and electronically recorded incriminating conversations.

A publication ban prevents CTV News from reporting any details about the officer, who is being called UCJ. The testimony is expected to last for about five days.

Nadia Gehl was shot and killed in Kitchener on Feb. 2, 2009. Her husband Ronald Cyr, along with Zdenek Zvolensky and Nashat Qahwash are charged with first degree murder in the case.

On Wednesday UCJ told the court how Cyr was befriended and his trust gained.

UCJ also said that Cyr ultimately described how he planned Gehl's murder and that Zvolensky and Qahwash ultimately carried it out.

During questioning by the Crown, UCJ also discussed day to day interactions with Cyr, and how the two became friends at training to become furniture salesmen.

The undercover officer was initially told the task of befriending Cyr would last only eight days, but in the end it continued for about five months.

Cyr and Zvolensky both took notes throughout the testimony, while Qahwash sat still.

On Tuesday, a specialist from Telus Communications testified about using cell phone towers to track the movements of the accused on the day before and after the alleged murder.

A firearms specialist was also brought in to interpret text messages sent between Qahwash and Zvolensky that were alleged to reference a gun.

UCJ's testimony is expected to continue on Thursday before the trial breaks for the weekend.