TORONTO -- A pathologist testifying at the trial of an Ontario woman accused of killing her severely disabled daughter told court Tuesday there was "no physical evidence" to suggest the girl had been smothered to death.

Dr. Michael Shkrum presented his views as the first witness called by Cindy Ali's defence lawyer.

His testimony followed weeks of evidence from Crown prosecutors who argue Ali murdered her 16-year-old by smothering her with a pillow and then spun an elaborate web of lies about a home invasion to cover up the crime.

Ali has pleaded not guilty to first-degree murder in the death of her daughter Cynara -- who had cerebral palsy and couldn't walk, talk or feed herself.

The teen died in a Toronto hospital in February 2011, two days after emergency personnel responding to a 911 call about a home invasion found her without vital signs.

An autopsy found Cynara's immediate cause of death was a brain injury from deprivation of oxygen, which was caused by cardiac arrest. That cardiac arrest, court heard, could have been caused by suffocation.

Shkrum prepared a report that reviewed that autopsy, as well as Cynara's medical records.

"I have no definitive signs or physical evidence that smothering has occurred," he said.

Cynara had a history of seizures and could have stopped breathing and suffered a cardiac arrest as a result of seizures, he said.

He also noted that Cynara's autopsy recorded a lung infection, which could also have been a factor in her death.

"We have physical evidence for aspiration pneumonia, we have clinical evidence for seizure disorder. It was well-documented in her life, we know it's associated with sudden death, and she had cerebral palsy," he said.

Smothering, on the other hand, is difficult to determine during an autopsy because there may be very few signs of it, Shkrum said.

Scrapes or bruises on a person's nostrils, lips, or tears inside the mouth, as well as pieces of material or fibre in the mouth are potential signs, he noted.

But no fibres were found in Cynara's mouth, and there was only a superficial bruise on the right side of her mouth which was likely caused by a tube placed there while she was in hospital, he said. A mark on her collar bone was likely from a neck brace, he added.

Under cross-examination, however, Shkrum agreed that Cynara being found with a blood-smeared pillow over her forehead and a towel over her neck "are not benign circumstances."

"You would agree with me, given the history and scene information, that smothering cannot be excluded in this case as a possible cause of a cardiac arrest," Crown prosecutor Rosemarie Juginovic asked pointedly.

"I cannot exclude that, no," said Shkrum.

Ali's lawyer sought to counter that exchange with one of his own a few minutes later.

"You found no evidence of smothering," Christopher Hicks pressed. "So smothering is hypothetical here."

"At least from the post-mortem exam," Shkrum said.

Later Tuesday, a pastor who counselled Ali and her family said the woman was a loving mother who was devastated by Cynara's death.

Sheela Duraisami, who was present when Cynara was taken off life support, said the disabled teen never came across as a burden to her family.

"Cindy was very, very affectionate and took care of Cynara, that was her life," Duraisami said.

Court also heard an intercepted phone call between Duraisami and Ali from January 2012 in which Ali tells the pastor police suspect her daughter's death was a homicide.

"Obviously she died of suffocation," Duraisami is heard saying, while urging Ali to keep telling the truth about the purported home invasion. "Your story is very consistent ... you found her like that."

The jury has heard that Ali was the primary caregiver for Cynara and was alone with her daughter on the morning she called 911.

Ali claimed two masked men in suits rang her door bell and pushed their way into her home demanding a mysterious package -- which they never found -- and harmed her daughter, court heard.

She claimed she saw one of the intruders hold a pillow over the girl's face, but later told police she only saw him with a pillow in his hands standing over Cynara, court heard.

Firefighters responding to the 911 call found Ali on the floor, with no injuries, and Cynara on a couch with no pulse.

Ali didn't move when asked to clear space for crews to conduct CPR on Cynara and didn't answer questions about her child, court heard.

Jurors also heard that firefighters didn't see wet footprints in the hallway of the home despite a light dusting of snow on the ground that day, and that drawers in several rooms were pulled open but their contents did not appear to have been rifled through.