A forensic investigator with the Montreal police has opened testimony in Luka Magnotta's murder trial by describing the discovery of a human torso in the trash outside the accused's apartment in 2012.

One of the first people at the scene, investigator Caroline Simoneau, told jurors her team searched through 31 garbage bags left outside the building in Montreal's Snowdon neighbourhood.

Simoneau is the first witness to testify at the first-degree murder trial of Magnotta, who lived in the building.

Earlier today, the court heard Magnotta has admitted to the acts underlying the five criminal offences he's charged with, including the killing of 33-year-old university student Jun Lin, but has pleaded not guilty, claiming he is not criminally responsible because of mental illness.

Magnotta pleaded not guilty to:

First-degree murder.

Committing an indignity to a body.

Publishing obscene material.

Criminally harassing Prime Minister Stephen Harper and other MPs.

Mailing obscene and indecent material.

Simoneau is the first of more than 60 witnesses expected to be called before the court in the six-week-long trial.

Along with photos of the garbage bags, Simoneau presented images of a canvas suitcase found at the scene, partially open.

Inside, she said, investigators found a human torso, missing its arms, legs and head.

The investigators also found a severed limb, articles of clothing, two knives, a shower curtain, scissors and a dead dog among the trash.

All of the evidence recovered was sent to a lab for analysis.

Not criminally responsible defence

Before Simoneau was called to the witness box, Quebec Superior Court Justice Guy Cournoyer gave the jury members instructions on their duty to the court.

"He admits the acts or the conducts underlying the crime for which he is charged. Your task will be to determine whether he committed the five offences with the required state of mind for each offence," he told the jury.

The Crown no longer needs to prove those facts, but needs to convince the jury that Magnotta is guilty of premeditated murder and should be held criminally responsible for his actions.

In his opening statement, Crown prosecutor Louis Bouthillier told the jury he intends to prove that Magnotta spent six months planning the alleged murder before it was committed.

He told the jury they would have to watch video footage that depicts evidence of the killing during the trial.

Magnotta's defence lawyer, Luc Leclair, told the jury he intends to prove that his client was not criminally responsible for his actions. He said Magnotta is schizophrenic, and was hearing voices and feeling persecuted before the events took place.

He also told jurors they would be hearing from Donald Newman, Magnotta's father, who also suffers from schizophrenia, and a number of health professionals who had treated Magnotta over the years.

He said he plans to present medical files and evidence of past diagnoses as evidence.

Lin's father present in court

The small courtroom where the trial is being held only has seats for a handful of media and members of the public. Three seats in the courtroom have been set aside for Lin's family.

Jun Lin's father, Lin Diran, right, stands with his translator outside the Montreal courthouse as the family's lawyer addresses the media. Lin Diran intends to be in court for the duration of the trial, the lawyer said. (CBC)

However, a small room off of the main court room has also been provided so that Lin's father can watch the proceedings and consult with lawyers and translators as the evidence is presented.

The lawyer representing Lin’s family said they are grateful for the accommodations made by the court.

Lin’s father, Lin Diran, is being assisted by three Mandarin-speaking translators. He came from China for the trial, but Lin’s mother and sister, who are still struggling to cope with the death, didn’t make the trip.

"The idea for them to attend the real trial was too much, so they have stayed at home in China," Daniel Urbas, the family’s lawyer, said outside of court.

"This is a lot of stress for them."

Lin’s father intends to be in court every day, Urbas said, adding that the team working with him is covering up graphic evidence as it comes on the screen and explaining the evidence to him verbally instead.

"Some people can’t understand why a father would want to see evidence like this, but it’s his choice. It’s his son," Urbas said.

"He decided that the way he wants to honour his son is to be here. But there’s also a personal interest. He wants to know what happened to his son, how it happened, who did it… and, the question may never be answered, but why — why did it happen."

The trial continues tomorrow morning with the conclusion of forensic investigator Simoneau's testimony.