Beverly Rowbotham may have been attacked by a stranger, not her husband, on the night she was brutally killed, a defence lawyer suggested Wednesday.

Tim Killeen raised the idea as he grilled RCMP DNA expert Sandra Korkosh about blood and other evidence the Crown has relied on in the lengthy second-degree murder trial of Mark Stobbe, a former political adviser in Saskatchewan and Manitoba.

The Crown alleges Stobbe killed his wife in the couple's backyard in St. Andrews, Man., then drove the body 15 kilometres north to Selkirk to make it look like she had been robbed.

Killeen asked Korkosh about mysterious DNA found on the straps and zippers of Rowbotham's purse, which was found beside her body in the family car.

The DNA was male, but did not come from Stobbe or the couple's two sons, Korkosh testified.

"Could it be that a male could have grabbed her purse strap and pulled?" Killeen asked.

"It's a possibility I can't say no to," Korkosh replied.

Crown's case based on DNA evidence

The Crown's case relies largely on DNA evidence. No one saw Rowbotham's death in Oct. 24, 2000, and no one has testified that they saw the couple argue. Neighbours reported seeing or hearing nothing unusual that night.

Stobbe told police he had fallen asleep while Rowbotham was out for a late-night grocery shopping trip, and awoke around 2:30 a.m. to find her still gone.

Her body was found a few hours later in the back seat of one of the family's cars. Her head had been bludgeoned.

The Crown has produced DNA evidence of small blood drops, hair clumps and tiny bone fragments, some of which has been shown to have come from the victim, in the couple's backyard and garage.

The Crown's theory is that Stobbe attacked his wife with a hatchet during an argument in the yard, carried her body to a car in the garage, drove to Selkirk and then bicycled back home to report her missing.

Killeen spent much of Wednesday questioning the DNA evidence. He said a small blood stain found on a fridge in the couple's garage, which contained DNA from both Stobbe and his wife, could have come from a mosquito.

"We know that mosquitoes transfer diseases … they take blood from people."

Killeen asked whether it was possible that a mosquito could have bitten both Stobbe and the victim, and then been swatted against the fridge, causing a small bloody mess.

"Theoretically, that is a possibility," Korkosh answered.

Stobbe had worked as a high-ranking adviser to former Saskatchewan premier Roy Romanow before moving to Manitoba in the spring of 2000 for a job with the newly elected NDP government of Gary Doer.