Prosecutors say they have audio-tape evidence of Vancouver's Charles Kembo killing one his four murder victims. (CBC)

Testimony at a Vancouver murder trial has revealed that a suspect in three murders allegedly committed a fourth killing while he was under intense police surveillance, and that police eventually heard the sounds of the final murder being committed

Charles Eli Kembo has been charged with four counts of first-degree murder following the slayings of his wife, business partner, girlfriend and step-daughter.

It's alleged that the killing of Kembo's 21-year-old step-daughter, Rita Yeung, in July 2005 occurred at the same time that police were tapping the suspect's phone, had audio-surveillance in his apartment and also a GPS tracking device and audio bug in his vehicle.

It was the listening device in Kembo's car that prosecutors say caught the sounds of Yeung's murder.

The Crown alleges Kembo drove Yeung around for nine hours, ignoring her repeated requests to be let out of the car, before slaying her.

'A muffled scream'

The court was told this week that police believed they could hear the sounds of her last moments alive.

"There is a sound, we suggest, is a muffled scream from outside the vehicle at about 1:43 a.m. And nobody hears from Rita Yeung again," the Crown said.

Police were not constantly monitoring the surveillance devices in real time, and only heard the events later as they listened to recordings.

The Integrated Homicide Investigation Team would not comment directly on the Kembo case because the trial is underway, but one officer admitted that surveillance could present difficult situations for police.

"In those extremely rare cases that something like that would happen, it's heartbreaking for the investigators, and for all police, quite frankly," said RCMP Cpl. Dale Carr.

One security expert told CBC News that police no longer have the resources to listen live around the clock to suspects under surveillance. Taps are now digitally recorded, transcribed, and reviewed later.

"They are facing things like budgetary restraints. Sometimes I wonder how these guys can function without sleeping," said Ozzie Kaban, of Kaban Protective Services in Vancouver.

Profited from murders, says Crown

Investigators said it was 24 hours after the fact that they heard what they believe were the sounds of Rita Yeung's murder. They allege that they then used GPS records to retrace the location of Kembo's SUV at the time, and found Yeung's body.

The Crown alleges that Kembo killed his mistress, Sui Yin Ma, whose body was discovered in November 2004 in a Richmond slough; that he killed his wife, Margaret Kembo, some time late in 2002, although her remains have not been recovered; and that he also murdered business partner, Ardon Samuel, who was found mutilated in a Vancouver park, in November 2003.

The Crown said it would argue that Kembo committed the murders to profit from credit cards, insurance policies, and other sources connected to the victims.

None of the charges against Kembo has been proven, as the trial continues in B.C. Supreme Court in Vancouver.