The 18-year-old died at the scene while the 10-year-old later died in hospital.
Toronto Councillor Paula Fletcher told CP24, a Canadian cable and satellite TV channel, that she heard that the gunman was emotionally disturbed.
Three more people were reported hurt in the attack. Authorities have not disclosed a motive.
One of the victims, a young woman, has died, but no word has been given out on the identity of the second victim.
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DeRozan had said emphatically in the past that he wanted to finish his career with the Raptors. "We spoke. I think maybe my mistake was talking about what we expected going forward from him".
Paramedics said numerous victims, including a child, had been rushed to trauma centres, however, there was no immediate word on their conditions. The suspect fled on foot and was found dead nearby.
He added that the shooter exchanged fire with police officers and died at the scene, though no officers were injured. "Two civilians were fatally injured", the Ontario Special Investigations Unit said. The injured ranged in age from 10 to 59, and suffered injuries ranging from serious to minor, Police Chief Mark Saunders said Monday.
John Tulloch said he and his brother had just got out of their vehicle when he heard about 20 to 30 gunshots.
Toronto Police Chief Mark Saunders refused to speculate on a motive but told reporters he could not rule out terrorism.
The incident began around 10 p.m.in the city's Greektown, an area home to dozens of restaurants, small businesses and family homes.
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Jordan Spieth was tied for the lead at Carnoustie as he tries to become the first repeat victor in 10 years at the British Open. I figured I'd have to go get that number. "I really didn't feel like I really made a bad swing until 18".
An adult woman was killed and 13 others were wounded in the shooting, which was carried out with a handgun.
She was said to be in a critical condition. Their conditions are not known.
Toronto Mayor John Tory arrived on the scene and thanked first responders for their efforts.
Paramedics, firefighters and police soon converged on the scene of the shooting in Toronto's east end, which has many popular restaurants, cafes and shops.
Police have yet to release any confirmed details of the late Sunday shooting.
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Another person posted a GIF of comic Steve Carell grimacing with the caption: "These Danica Patrick jokes not hitting.at all". She also joked that the football star probably has fewer tickets - adding she once got pulled over three times in three days.
Police responded to reports of an incident in the city's Riverdale neighbourhood around 10pm Sunday night (local time).