A B.C. Supreme Court jury has begun deliberations at the trial of Kelly Ellard, to decide whether she helped kill teenager Reena Virk in 1997.

Ellard, now 22, is charged with the second-degree murder of the 14-year-old, who was killed in the Gorge Waterway in suburban Victoria after being swarmed and beaten by a group of teens.

Mr. Justice Robert Bauman spent Friday morning instructing the jury on how to consider the evidence in the five-week trial. He paid particular attention to the testimony of Warren Glowatski, the only person convicted of murder in the death.

Bauman told the jury they must be careful of Glowatski's testimony that Ellard delivered the final, fatal assault on Virk. The judge also warned jurors to look for other testimony that supports that testimony.

He said that the jurors must ask themselves whether Glowatski's evidence has the ring of truth to it.

Glowatski had told the court that he took part in the swarming, which left the teen semi-conscious. He said he and Ellard returned to attack Virk again, kicking and punching her unconscious.

He also said Ellard dragged Virk into the water and held her head underwater until she drowned.

The judge said that if the jurors have reasonable doubts that Ellard committed the fatal assault, they must acquit.

But he said that if they believe Glowatski and Ellard participated in the second beating, it doesn't matter who drowned Virk as both would be guilty of murder.

Glowatski was convicted of Virk's murder in 1999 and is serving a life sentence with no parole for seven years.

Ellard looked relaxed throughout the morning, and smiled and waved to her parents in the courtroom before being led out by a sheriff.

She didn't testify at the trial.