TORONTO (AP) _ Canadian police said Tuesday they still don't know what caused the death of a student from Beijing who was last seen alive on a webcam by a friend who witnessed her struggling with a man before the camera was abruptly shut off.

Police said the body of 23-year-old York University student Qian Liu was found Friday in her apartment in Toronto a few hours after her friend in China witnessed the attack. She was naked from the waist down and there were no obvious signs of sexual assault or trauma that would have led to her death, police said.

An autopsy failed to show the cause of death and it could be weeks before the results of toxicology tests are known, Police Sgt.

Frank Skubic said. He said the death has yet to be classified officially as a homicide, though police were investigating it as such.

Liu was chatting with her male friend by webcam at about 1 a.m. Friday when an unknown man knocked on the door, police said.

The friend reported seeing a struggle break out between the two before Liu's webcam was shut off. The friend contacted other friends in Toronto who in turn called police.

``We have spoken to this online witness by telephone,'' Skubic said.

Liu's laptop computer, webcam and mobile phone are missing, Skubic said. He said the online chat was a live streaming camera and was not recorded, though investigators were trying to figure out if there was any way they could recover it.

The witness told police the man who assaulted Liu appeared to be in his 20s with a muscular build.

Skubic said it was not clear if Liu knew her attacker. He said a former roommate was interviewed as a person of interest, but he declined to call the man a suspect.

Skubic said Liu's family was expected to arrive in Toronto at the end of the week. Skubic said she lived alone and had been at York since last September.

``Our entire community mourns the loss of a promising young student,'' York University President Mamdouh Shoukri said in a statement.

York student James Gibson, 24, lives next door to Liu's apartment but said he had never met her. The neighborhood, just south of the campus, is known for crime, he said.

``I've heard there's been a lot of criminal activity going on around here, a lot of robberies ... but I never thought there would be a death,'' Gibson said.

Shanta Poonai, who lives a few doors down, said the neighborhood is mostly students who often hold parties.