By Holly Yan and Madison Park, CNN

(CNN) -- The killer's family had tried to get him help for years.

But nothing helped mitigate Faisal Hussain's mental health issues, his relatives said. Now they -- along with many others in Toronto -- are devastated by Hussain's shooting rampage that killed two people and wounded 13 others late Sunday night.

"Our hearts are in pieces for the victims and for our city as we all come to grips with this terrible tragedy. We will mourn those who were lost for the rest of our lives," Hussain's family said in a statement.

"Our son had severe mental health challenges, struggling with psychosis and depression his entire life. The interventions of professionals were unsuccessful. Medications and therapy were unable to treat him. While we did our best to seek help for him throughout his life of struggle and pain, we could never imagine that this would be his devastating and destructive end."

Investigators are still trying to determine why Hussain, 29, opened fire on Toronto's Greektown neighborhood as families dined at Greek restaurants.

Toronto Police Chief Mark Saunders said authorities are investigating "every possible motive, including terrorism," for the attack. But the motive may be elusive, as Hussain was also killed. Police have not said whether he was killed by responding officers or by a self-inflicted gunshot wound.

Slain victims were 10 and 18 years old

The gunman killed 18-year-old Reese Fallon of Toronto and a 10-year-old girl who has not been publicly identified.

Fallon graduated from Malvern Collegiate Institute just last month, said John Malloy of the Toronto District School Board.

The teen "was highly regarded by staff and loved by her friends," Malloy said in a statement.

Fallon was going to attend McMaster University and was known for her work with the young Liberals political group, CNN partner CBC reported.

The 13 people wounded range from ages 17 to 59, police said.

Gunman's friend is stunned by the news

Aamir Sukhera, a lifelong friend of Hussain's, told CNN partner CTV that he couldn't believe what happened. He said Hussain, a Toronto resident, was "polite, respectful, humble" and quiet, and didn't really talk to many people.

"He seemed fine," Sukhera said. "I can't put two and two together. I mean, I can't believe it's him."

The deadly melee in the popular Toronto neighborhood has led to increased scrutiny of the city's gun violence.

Just two days before the shooting, Toronto had deployed 200 more officers on the streets of at-risk neighborhoods between 7 p.m. and 3 a.m. in an effort to reduce gun violence.

After the rampage, Toronto Mayor John Tory said the city has "a gun problem in that guns are readily available to too many people."

"The police are doing their best, but they're operating under extraordinarily difficult circumstances to deal with these guns," the mayor said. "We'll see what they conclude from this case, but it's evidence of a gun problem, clearly."