TORONTO -- A pathologist told a York University student's murder trial Monday that she died from neck compression, going a step further than another pathologist who said he couldn't arrive at a definitive cause of death.

The first pathologist listed Qian Liu's cause of death in April 2011 as unascertained, but said the best explanation is mechanical asphyxia, which could be caused by neck or chest compression or when the airways are otherwise blocked.

Dr. Christopher Milroy, who did a peer review of the first pathologist's report, testified Monday that in his opinion the cause of death is mechanical asphyxiation due to neck compression.

Other autopsy findings such as bruising in the neck muscles and small hemorrhages in the eye are consistent with that cause of death, Milroy said. The hemorrhages, known as petechiae, are a result of blood supply to the head being obstructed, Milroy said.

"We don't know exactly how long, but it takes some time to produce petechiae," Milroy testified. "It certainly requires firm pressure and for some time -- a number of seconds -- before you get petechiae."

There were no external injuries on the 23-year-old woman's neck, but it's possible to compress the neck without leaving external marks, particularly if a broad hand or arm is used, he testified.

Liu -- who was found dead in her off-campus apartment, mostly naked and face down on the floor -- had been chatting via webcam with her ex-boyfriend in China when he saw an intruder force his way into her room.

There was a struggle and Liu was pushed down, the ex-boyfriend testified. She was saying "no," but after he heard two muffled bangs there were no more sounds from Liu, the ex-boyfriend testified.

After a period of silence he heard heavy breathing then the lower half of a naked man appeared in front of the webcam and the computer was turned off, the ex-boyfriend testified.

Brian Dickson, who was a tenant in the same building, has pleaded not guilty to first-degree murder, but his lawyer has told the jury he'll be urging a finding of manslaughter.

Defence lawyer Robert Nuttall raised several alternate scenarios for Liu's cause of death.

He asked the pathologist to envision a large man sitting on Liu's chest, which would compress her airway, and her head positioned against the wall in such a way that her neck was cricked, which Nuttall suggested could account for internal neck muscle bruising.

Milroy, a veteran forensic pathologist, said he actually dealt with a similar case in Ontario, and no bruising was found in the neck.

The jury has heard that semen was found on Liu's abdomen and groin area. Swabs for DNA were also taken from her breasts and Nuttall asked the pathologist that if the source of that DNA was seminal, would that confirm the presence of somebody on or near her chest. Milroy said it wouldn't necessarily indicate that.

Nuttall also asked whether heart disease, overstimulation of a nerve in the neck or commotio cordis, which is sometimes seen in youth athletes who drop dead after a blow to the chest because it interrupts the firing mechanism of the heart, are possible causes of death.

Milroy dismissed the scenarios because he would not expect to see the combination of petechiae and bruising of the neck muscles in those cases.

"Never say never and never say always, but there are features in this case that are against it," Milroy said about the nerve overstimulation scenario.