This image, taken from police surveillance footage released to the media, shows Carol Berner speaking with the undercover officer. (CBC)Police are crediting an unusual sting operation for helping to obtain a guilty verdict in the 2008 drunk-driving death of a four-year-old girl.

Carol Berner, 58, of Delta, B.C., was found guilty on Tuesday of four charges, including impaired driving causing death.

In May 2008, Berner lost control of her car while driving on a rural road in Ladner, south of Vancouver, hitting Alexa Middelaer, 4, and her aunt, Daphne Johnson. The pair had been feeding horses outside a farm fence on the roadside.

Middelaer died after being taken to hospital, while Johnson spent a month in hospital with serious injuries.

Delta police say it was an extraordinary drunk-driving case that required unusual investigative techniques.

"That was an option that was available to us within the provision of the law and we took that," said Delta Police Department spokeswoman Sgt. Sharlene Brooks.

During a three-month-long sting, undercover police officers befriended Berner, even taking her to a Madonna concert.

Berner eventually admitted to the undercover officer that she drank three glasses of wine before the incident, as police secretly captured the confession on videotape.

Cost of investigation not released

David MacAlister, an associate professor at SFU's school of criminology, said it's unusual for the expensive technique to be used in an impaired-driving case. (CBC)Experts say using an undercover sting for a drunk-driving case is relatively rare, as the technique is usually used in murder investigations.

"It is fairly ground-breaking. It's the first time I've heard of it being used outside the organized crime world, and I think it's certainly another tactic that police can and will be using," said David MacAlister, an associate professor at SFU's school of criminology.

"Police worry about the courts cutting back their powers or limiting their ability to conduct investigations, so I think they're going to be celebrating when they use a new tactic that works."

MacAlister said it is an expensive technique, which means it will likely only be used in serious impaired-driving cases.

Delta police have not said how much the investigation cost, saying only it was not a question of money.

"Was it worth it? What price tag do you put on four-year-old little Alexa Renee Middelaer?" Brooks said.

It is expected Berner will be sentenced on Nov. 8.