Joe Warmington of The Toronto Sun reported that Hussain apparently had been spoken to by authorities about his online activities.
"People should rely on the information that is going to be forthcoming over time, including today, from the Toronto Police Service", he said.
He added that the police are investigating "every possible motive, including terrorism". "It's way too early to rule out anything".
According to a source in the police department who spoke with Canada's Global News, Hussain was "fascinated with death and explosions", liked "replica handguns", and strongly identified with Batman's archenemy the Joker. His parents made daily hospital visits to see his brother, who has been in a coma for almost a year and a half following an accident.
"Tonight, a despicable act has been perpetrated on the Danforth in our city". In April, a driver deliberately ploughed his white Ryder rental van into a lunch-hour crowd, killing 10 people and injuring 15 along a roughly mile-long (1.6-km) stretch of sidewalk thronged with pedestrians.
The said he had battled "severe mental health challenges, struggling with psychosis and depression his entire life".
Hussain's family said he had suffered since infancy from serious mental problems, which treatment and medication were unable to overcome.
Among those killed, was 18-year-old Reese Fallon, a recent high school graduate who volunteered for Canada's Liberal Party and was to attend McMaster University in the fall. The family said they were shocked at Hussain's "senseless violence" and expressed their condolences for the families of the shooting victims.
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Two females, an 18-year-old and a 10-year-old, were killed and after exchanging gunfire with police, the shooter also died, but it is unclear if he was shot by police or fell victim to a self-inflicted gunshot wound.
Around the Thorncliffe Park apartment Hussain shared with his parents, he would smile in recognition, but rarely spoke to neighbours and did not seem to have friends or romantic interests, the neighbour said.
A large part of Danforth Avenue reopened for business, after police completed their forensic search of the crime site.
Just hours after the shooting in Toronto, police arrested a man who tried to stab a member of the ceremonial guard outside Parliament in Ottawa.
The man was quickly arrested and no one was injured in the incident, the Canadian defense ministry said.
Ironically the chants of "Toronto the strong" and "Toronto the resilient" ring hollow at this time when terrorist attacks are becoming the norm.
The deadly melee in the popular Toronto neighborhood has led to increased scrutiny of the city's gun violence.
Gun violence is nearly unheard of in the area and in Toronto, and the realization that a neighbor had gotten a handgun was deeply unsettling to many of Hussain's neighbors. Handguns and other restricted firearms require passing an additional course.