DNA evidence links samples to accused in Surrey murder trial, jury hears

Forensic biologist Janny Lau testified Thursday at Adam Troy Mann’s second-degree murder trial in B.C. Supreme Court in New Westminster. Mann is accused of fatally stabbing Tori Dunn in June 2024 during an alleged home invasion in Surrey’s Port Kells neighbourhood. He listened quietly through the day’s proceedings, as did many of Dunn’s family and friends, including Trafford Wace, her boyfriend at the time of her death and the person with whom she shared a home. Wace testified earlier that he noticed a kitchen knife missing from their home after the alleged break-in.

Lau works at the RCMP National Forensic Laboratory. After showing the court a PowerPoint presentation on how DNA testing works, she described comparing evidence from the case — a bloody kitchen knife, a hoodie and a swab from a cellphone — with known DNA samples from Dunn and Mann. She said her work is to interpret DNA profiles, determine from known samples how many people are represented in a sample, and carry out any necessary preparation, measurements or inclusion analysis to provide statistical results. Testing looks at specific locations in DNA called short tandem repeat regions, which contain sequences that repeat many times.

Lau said blood on the blade of a large kitchen knife found in the neighbourhood near the Port Kells home was a highly probable match to Dunn. The DNA profile from the centre of the blade, area AA, matched Dunn’s known sample, she told the court. The likelihood that the blood came from someone other than Dunn was one in 290 quintillion. The court heard that a quintillion is a one followed by 18 zeros. Testing also confirmed DNA from three people on the knife handle. Compared with Mann’s known sample, the results showed it was 2.7 quadrillion times more likely that one of the DNA profiles on the handle came from Mann than from a random member of the population. Lau confirmed that a quadrillion is a one followed by 15 zeros.

Lau also testified about a grey hoodie found outdoors near Dunn’s home. Wace had previously testified that he saw the suspect wearing a similar garment. Lau said two DNA profiles on its sleeve were likely matches to Dunn and Mann. A swab taken from Dunn’s iPhone, also found outdoors in her neighbourhood, was a highly likely match to Mann’s known DNA sample, Lau said. The results showed it was 12 billion to 17 billion times more likely that the DNA came from Mann than from a random, unknown member of the population.

During cross-examination, defence lawyer Paul McMurray focused on the population databases the RCMP used for DNA analysis. Lau testified that in 2024 the RCMP relied on the Canadian Caucasian population database. The federal police force has since switched to a stratified population database that includes several ethnicities. Lau re-analysed all collected evidence samples using the stratified database, and the results were similar to those from the original analysis using the Caucasian database, the court heard. Asked whether she was concerned about the minor numerical differences between databases, Lau said she was not.

Lau said the results were “numerically slightly different,” but the verbal likelihood ratio in each instance was “very strong.” She said “very strong” is the strongest category on the verbal scale, describing it as a one-million-per-cent probability. McMurray asked whether any of the databases included people with genetic backgrounds similar to Mann’s; the court heard he is half African-American and half Indigenous. “If you don’t have any samples from persons of a similar genetic background, you’re comparing apples to oranges,” McMurray said. Lau replied that the characteristics of any DNA sample would “be observed within the mixture.” She explained that she calculates how often someone in the population would have a profile like the one found.

One more witness is expected to testify Friday, Oct. 16. B.C. Supreme Court Justice David Layton is presiding. The trial began Oct. 5 and is expected to last about three weeks.