The two Vaughn sisters, 12-year-old Abigayle and 11-year-old Cassandra, were sitting side-by-side in the back seat of the family's SUV when someone fired bullets into each of their heads and torsos, a forensic pathologist testified Wednesday.

Their brother, 8-year-old Blake, was shot in the head and under the arm -- the latter likely the result of him raising his arm in a defensive motion, Dr. Larry Blum said.

The murderer is either their father, Christopher, eager to shed his boring suburban existence for a life in the Canadian wild, or their mother, Kimberly, suicidal over her crumbling marriage and perhaps under the influence of a "toxic" level of an antidepressant drug.

Christopher Vaughn, now 37, is on trial for the murders, which occurred in the early morning hours of June 14, 2007, on a lightly traveled road near Channahon.

But Blum helped bolster the defense's case that Kimberly Vaughn is the actual culprit by acknowledging the gun shot fired into her brain from below her chin could have been self-inflicted.

Blum also testified that Vaughn had more than 500 mg of nortriptyline in her blood at the time of her death, when the daily maximum dosage is 150 to 200 mg. Side effects for the drug, typically used as an antidepressant, include the potential for suicidal thoughts, especially in children, young adults and adults under the age 24.

Earlier testimony indicated that Vaughn, 32 at the time of her death, had been prescribed the drug to treat migraine headaches.

Blum was not the pathologist who did the autopsies in 2007, but was called by prosecutors to review the work done by Dr. Bryan Mitchell, who died in 2010, and to testify about the findings.

Given the "stippling" -- small dot-like injuries created by gun powder and other debris released when a gun is fired -- near the head wounds and some of the body wounds, Blum said he believed the gun was no further than 18 to 24 inches away from each child before being fired.

While the head wounds likely killed all three, the secondary shot in the torsos contributed to their deaths, he said.

As for Kimberly Vaughn, her head injury would have almost immediately "incapacitated" her and death would have followed quickly, Blum said.

Blum and Mitchell were required to determine the cause of death -- in this case, gun shot injuries -- but they were not required to identify the "manner" of death. That is the purview of the Will County coroner, who can declare it natural, homicide, suicide, accident or undetermined.

Despite that, Blum said he could offer his opinion.

"Based on what I know ... I cannot rule out suicide," he said.

Prosecutors, anticipating attorney George Lenard's plan to paint Kimberly Vaughn as suicidal, have had several friends and family members testify in the last three weeks about Vaughn's approach to motherhood and her demeanor in the days prior to her death.

"She was a very friendly, outgoing, bubbly personality," said Reina Carrasco, who met Vaughn when they started working at their Oswego subdivision's clubhouse about two months before the shooting.

Carrasco said they became fast friends, seeing each other three or four times a week, in part because their youngest and oldest children were the same age.

Vaughn was the kind of mother who got in the pool and played with her children rather than watching from the sidelines, she said, and she would help them do homework when they'd drop by to see her at the clubhouse.

Anthony Bridges, who also worked at the clubhouse, had similar memories.

"She was a very lovely mother," said Bridges, who recalled her constantly checking on her kids when they were in the pool. "I thought she was an outgoing person, always looking forward to expanding her career and making things better for her children."

But he also remembers one discussion they had about her career goals in which she told him she might join her husband in a private investigation business once she completed her college degree. The only thing holding her back was having to have a weapon.

"She wasn't very comfortable with guns," he said. "She didn't think they were the right way to resolve a situation."

Friend Alyson Mals, who worked for the development company that sold the Vaughns their house, recalled Kimberly Vaughn dropping by the sales office to talk, sometimes for hours at a time. Vaughn helped counsel Mals as she went through a broken engagement, recalling in an email the turmoil that surrounded her own wedding 13 years earlier, in part because Vaughn was pregnant when she wed.

But she also acknowledged that Vaughn seemed "bored" to her, and maybe a little lonely because her husband traveled so much for work.

That said, Vaughn never spoke ill of her husband, said Lynda Vena, whom Vaughn befriended after she started going to Vena's hair salon in September 2006.

"She felt bad that he had to work so much," Vena said. "Kim more or less idolized him."

Here are the stories from the previous days of testimony:

Former Stripper Testifies in Vaughn Case

Vaughn Planned New Life in Canada, Email Friend Says

Gun Fired Less than 6 Inches from Vaughn

Vaughn Admits in Police Video that Wife Shot Him

Vaughn Told He 'Slaughtered' Family by Cops, yet Displays No Emotion

'Your Wife and Kids are Dead': Police to Vaughn

Jury Won't Hear Cop's Charge that State Ignored Murder-Suicide Theory

Vaughn Spent Thousands at Topless Club Days Before Family Murdered

Vaughn Worried about Boots, Jacket, but Not Wife and Kids, Witnesses Say

Kimberly Vaughn 'Happy' in Days Before Murder, Witnesses Testify

Grim Death Scene Photos Don't Rattle Chris Vaughn on Murder Trial's First Day