Prosecutors allege that Entwistle shot his wife and daughter on Jan. 20, 2006, after becoming despondent over mounting debt and dissatisfied with his sex life. The suspect flew to his native England the day after the killings and was arrested in London three weeks later.

Entwistle told police he came home from running errands and found his wife and daughter fatally shot in their bed in the master bedroom of their rented house in Hopkinton, where they had moved 10 days earlier.

Entwistle's lawyer, Elliot Weinstein, aggressively cross-examined Dygan about work she and other scientists did at the crime scene. In his questions, he tried to undermine investigators' credibility by highlighting the fact that police initially did not realize that Rachel Entwistle had been shot in the head.

At first, police noticed a wound above her left breast, but an autopsy later revealed that she died of a gunshot wound to the head. The post-mortem examination showed that Lillian Entwistle died of a gunshot wound to the chest; investigators said that bullet passed into Rachel Entwistle's chest.

"You didn't notice that she had been shot in the middle of her forehead at first?" Weinstein asked.

"This was the careful and meticulous work, and yet nobody saw that that evening, did they?"

"That's correct," Dygan replied.

Another state police chemist, John Drugan, testified that no gunshot residue was found in the family's car, a BMW that Neil Entwistle left at Logan International Airport when he flew to England the day after the killings. Gunpowder residue also was not found on the car keys or on a blue sweater found in the car, Drugan said.