Warning: This article includes graphic content that may disturb some readers.

MONTREAL -- A screen grab of a severed head convinced a good friend of Jun Lin that the Chinese engineering student was dead, Luka Rocco Magnotta's murder trial heard Tuesday.

Dong Dong Xu testified that his search for Lin in late May 2012 had turned up nothing when he first heard about a so-called murder video circulating online.

Armed with the video title, "One Lunatic, One Ice Pick," Xu went online and was immediately confronted with a gruesome still photo of a severed head that told him what he needed about his missing friend.

"I identified him instantly," Xu calmly told the jury as he detailed his frantic search for Lin.

Xu testified he was pressed into action on May 27, 2012, after a call from Lin's ex-lover, Feng Lin, who was abroad and hadn't heard from the victim in nearly 60 hours.

Xu said he first went to Jun Lin's apartment in Montreal and then a convenience store where he worked part-time. He immediately called police after the store owner told him Lin hadn't turned up for two shifts.

"I was afraid something unpleasant happened, so I called the police," said Xu, who had known Lin as Justin since meeting him in September 2011.

Xu continued his search at Concordia University, where the 33-year-old victim was enrolled as a student. He also called the Chinese Consulate in Montreal to report a missing person.

At Feng Lin's behest, he then went looking through Jun Lin's computer for clues. Lin's school bag was in the corner of his living room and his cash and personal identification were at home.

It was on May 29 that Xu heard people talking about an online video depicting what he called the "murdering process."

"I saw a severed head on the Internet but I didn't look at the video at first," he said. It was only later that day that he glanced at the video, which he described briefly for the jury.

"At the very beginning, a living person bound to the bed, a male, blindfolded, struggling," Xu said, adding the person was not Lin. The Crown has said the first 53 seconds of the video contain images of a different person.

In later scenes, a body was severed and there were images of someone with a knife trying to cut pieces off. A dog could also be seen licking severed body parts.

"I didn't watch the video again after that," Xu said, adding that one Montreal police officer told him he'd never seen a video of a crime scene like that in his career.

Lin was last seen alive walking into Magnotta's apartment building on the night of May 24, 2012. His torso was discovered in trash behind the building on May 29.

Magnotta, 32, has pleaded not guilty to five charges. He has admitted to the acts he's accused of in Lin's death, but is arguing he is not criminally responsible by way of mental disorder.

The Crown contends the slaying was planned and deliberate and that evidence suggests it was planned up to six months in advance.

A former client of Magnotta took the stand late Tuesday, describing how he'd paid for the accused's services as an escort.

Thomas Murphy, a 69-year-old retiree and former U.S. naval officer, said he had five encounters with Magnotta between March and May of 2012. Montreal police tracked him down using phone records.

"Plain Jane gay sex, this is about the best I can describe it," Murphy said of his encounters with Magnotta, which included no bondage, at $150 each.

Murphy said what he liked about Magnotta was that he didn't drink or do drugs. Magnotta went by the name Nathan22 on the gay escort site before telling his client he was called "Luke."

The witness described Magnotta as a "handsome, young guy" who was looking to make some cash.

It was Magnotta's appearance that caught Murphy's attention: short dark hair, well-shaped abs and a body that was not too muscular.

Murphy's first reaction in court on Tuesday was that he didn't recognize the person sitting in the prisoner's box because Magnotta's appearance has drastically changed since 2012.

The witness told the court he didn't remember much of his conversations with Magnotta, as talk wasn't the primary reason for their get-togethers.

Murphy said he'd heard about the slaying before leaving Canada, but didn't put it all together until Magnotta's picture was on the news in the United States following his arrest.

"I said 'holy crap! That's the guy I've been seeing,"' Murphy said. "I was scared out of my wits. To this day, I haven't seen another escort since."

Murphy called his dealings with escorts "one of the weak moments in his life" but said Magnotta was good for his ego.

"I felt like someone was being affectionate to me and that's what I was craving at the time," he said under cross-examination.

Earlier on Tuesday, a Montreal police homicide detective wrapped up her testimony.

Defence attorney Luc Leclair questioned Claudette Hamlin about surveillance videos of Magnotta seized by authorities at a Paris bus station and an Internet cafe in Berlin where the accused was arrested on June 4, 2012.

The jury watched Magnotta walking into the cafe. About 90 minutes later, an employee is seen motioning to Berlin police on the street. They enter, arrest Magnotta and leave with him in tow.

The charges against Magnotta are first-degree murder; criminally harassing Prime Minister Stephen Harper and other members of Parliament; mailing obscene and indecent material; committing an indignity to a body; and publishing obscene materials.

The trial resumes Wednesday.