The gun used in Sunday's deadly rampage on Toronto's Danforth Avenue has been traced to the United States and U.S. authorities are helping narrow down its origin, CBC News has learned from a police source.

The source said there is reason to believe that the gunman, Faisal Hussain, 29, might have obtained the firearm used in the attack from his older brother, who is in a coma in hospital.

The older brother is known to Toronto police for alleged ties to a Thorncliffe Park street gang, the source said.

Hussain's family and friends have said he was suffering from depression and psychosis, but police would not confirm any details of his medical history, citing privacy legislation. A police source did confirm, however, that Hussain had prior contact with authorities and that it involved mental health problems.

A source close to Hussain's family told CBC News on Tuesday that Hussain, who attended high school at Toronto's Marc Garneau Collegiate Institute and Victoria Park Collegiate Institute, was apprehended twice by police while he was under age 18.

Toronto police searched Hussain's apartment in the Thorncliffe Park neighbourhood on Monday afternoon, but have said nothing about a possible motive for the shooting.

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According to the police source who spoke with CBC News, investigators recovered a firearm and a computer from the apartment.

Surveillance video captured gunman walking past a business on Danforth Ave.:

Faisal Hussain captured on video walking past the Burger Stomper restaurant on the night of the deadly mass shooting. 0:34

No evidence linking shooter to ISIS, police say

Toronto police said Wednesday they have "no evidence" that Faisal Hussain's shooting rampage was connected to ISIS.

"At this stage, we have no evidence to support these claims," said Chief Mark Saunders.

Reuters had previously reported that the Islamic State in Iraq and Syria (ISIS), through the group's AMAQ news agency, had claimed responsibility for the attack, but did not provide any evidence to support the claim.

CBC News could not verify the legitimacy of the ISIS claim.

Asked Tuesday if there was any indication Hussain had expressed support for any extremist groups, a spokesperson for the Ministry of Public Safety downplayed any national security concerns.

"There is no national security nexus at this time," Hilary Peirce said in an email to CBC News. Peirce added that local police will continue to lead the investigation.

Hussain did not have a criminal record and his prior contact with police did not involve a risk to public safety, according to Toronto Police Service spokesperson Meaghan Gray

Asked if Canada's spy agency, CSIS, is involved in the investigation, Gray would not comment specifically but said police routinely call on other law enforcement agencies for assistance.

"This case is no different."

Aamir Sukhera was a close friend of Faisal Hussain and says he knew he had mental illness, but he thought he was doing fine. 0:12

While the search of the apartment is complete, police interviews with witnesses continue as does the work of creating timelines, compiling background information and other digging, including online searches, Gray told CBC News.

"This work takes time," she said, adding that nothing will be said about a motive until the necessary evidence is obtained.

"We are still in the early stages of this investigation."

Monica Hudon, spokesperson for Ontario's Special Investigations Unit, said in an email on Wednesday that an autopsy on Hussain was completed on Tuesday, but no results will be released until the SIU's investigation is over.

The SIU is probing the circumstances surrounding Hussain's death.

'Post-mortem results are considered to be evidence, and the SIU does not comment on evidence while an investigation is ongoing," she said.

The SIU investigates all reports involving police where there is death, serious injury or allegations of sexual assault.