"Despite intensive searches ... nothing allows us to establish how they met, when or why," Montreal police Det-Sgt. Claudette Hamlin testified.
The veteran homicide investigator says that included exhaustive checks of phone records and the online activity of both men, she said.
Police even combed through 7,484 photos found on Lin's computer but found none of Magnotta.
Magnotta, 32, has pleaded not guilty to five charges. He insists he is not criminally responsible in the Chinese student's death because of mental disorder.
The trial resumed Monday with more surveillance video from Magnotta's apartment building that shows him leaving for the last time on May 26, 2012.
He can be seen wearing a Mickey Mouse T-shirt, holding a carry-on style suitcase and getting into a taxi that is visible in the reflection of the glass front door of the building.
That video was captured two days after Lin was seen walking into the building with Magnotta at 10:16 p.m. on May 24, 2012. Lin, 33, was wearing a yellow T-shirt, shorts and a baseball cap. He was never seen alive again.
Just a few hours later, Magnotta was caught on the same cameras, wearing Lin's T-shirt and baseball cap while methodically putting trash in the basement of the building as well as carrying it outside over the next day. Magnotta comes and goes, sometimes stopping to look at his appearance in a mirrored wall in the lobby.
Lin's torso was found on May 29, 2012, stuffed into a suitcase behind the apartment building. Other body parts were also discovered in trash in the same area.
A baseball cap similar to Lin's was found among Magnotta's personal effects when he was arrested in Berlin in early June of that year. Another item retrieved was a dark wig that Hamlin said cost $1,700.
Hamlin said police also found a few receipts in the trash: one for Canada Post and one for bedding from Zellers that was dated May 25, 2012.
She told the jury that while the apartment had been thoroughly cleaned, the forensic lighting revealed plenty of blood.
On Monday, the jury watched video of another individual who had been to Magnotta's apartment a week before Lin's slaying.
That person, who has not been identified by investigators, appears in the first 53 seconds of the so-called murder video the jury has not yet seen.
Hamlin said police became aware of the second individual only in March 2013 while studying the contents of a video camera found in the trash.
That security video shows a man walking into Magnotta's apartment on May 18, 2012, and leaving the next day, with Magnotta holding his arm to help him down the stairs.
The jury spent the rest of the day looking at other security footage — from the airport in Montreal and from Charles de Gaulle Airport in Paris; from a hotel in the French capital; and from a bus station in that city as Magnotta waited to board a bus for Berlin on May 31.
Authorities in Berlin arrested Magnotta on June 4, 2012.
Several details emerged during defence attorney Luc Leclair's cross-examination of Hamlin, with the jurors learning that:
— Magnotta travelled to Europe under his own identity, although he was wearing the dark wig for much of the trip.
— Expedia records showed the airline ticket to Paris was purchased at 4:38 a.m. on May 25, just hours after Lin and Magnotta entered the building together.
— Magnotta changed his cellphone on the same day, a phone that stopped being active when he left the country the following day. There were no incoming calls to the number.
— The accused made several calls on May 25 to a number in the 705 area code, which covers much of northeastern and central Ontario.
While the native of Scarborough, Ont., admits to causing the acts he's accused of in Lin's death, his lawyer has said his client suffers from schizophrenia and was diagnosed with borderline personality disorder shortly before the slaying.
The Crown contends the killing was planned and deliberate.
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 material.
The trial continues Tuesday.
Note to readers: This is a corrected story. A previous version stated evidence was presented Friday, but the court did not hear evidence that day.
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