Officers pleaded for witnesses to come forward while promising to stand guard over those left shaken and fearful by the attack that killed two people and wounded two dozen Monday night.
About 200 people were at the barbecue held outside a community housing complex when two gunmen opened fire after an altercation in what police are calling the worst incident of gun violence in the city's recent history.
Police said they suspect gangs were involved in the shooting that killed 14-year-old Shyanne Charles of Toronto and 23-year-old Joshua Yasay of Ajax.
Twenty-three others were sent to hospital with gunshot wounds, including a 22-month-old child who was grazed by a bullet and is in stable condition, police said. One person was trampled in the ensuing stampede.
Some of the wounded, aged between one and 33 years old, were treated and released. One man remained in critical condition in hospital Tuesday afternoon, police said.
Messages of grief quickly appeared on social media even as some hinted the deaths could spur more violence.
"The hood gained another angel to look over us, RIP," one person wrote on Twitter.
Police Chief Bill Blair said police have received "some co-operation" from witnesses at the scene and called on all community members who attended the party to help with the investigation.
Police also asked for anyone with video or photos of the incident to send them in, a strategy they have used in similar investigations in the past.
"I'm here today to make an appeal through you all, to the many hundreds of people who were present at the event yesterday who may have some information relevant to this investigation to come forward," Blair said.
"We are very concerned not only with the quick resolution and solving of this crime but the potential for retaliatory violence, which we often see in this type of event," Blair said.
Police said one injured "person of interest" is in custody.
The incident took place before 11 p.m. Monday on Danzig Street near Morningside Avenue and Lawrence Avenue East.
The street was still strewn with empty water bottles and other remnants of the party on Tuesday, as swaths of police tape surrounded the area.
Shannon Longshaw said the party, which she helped organize, started as an afternoon barbecue for neighbourhood kids. But even as the crowd grew later in the day, she said the festivities stayed friendly — until shots suddenly rang out.
"There was no fighting, nobody got into an altercation, none of that," she said Tuesday morning, after spending much of the night being questioned by officers.
Bruce Strachan has lived in the area his whole life, but said he's now thinking of moving away.
"This is shocking," he said, watching police comb over the scene.
"I've got my two granddaughters living with me here. I brought them up from Trinidad to be safe from this type of violence and it's right around my corner."
Tears shone in one man's eyes as he recalled walking out of a friend's house and into the crowded celebration just moments before violence erupted.
"I saw people running, I heard 'crack, crack' and I hit the ground," said the man, who did not want to be identified out of concern for his safety.
"It was like a Quentin Tarantino movie," he said, calling it the most frightening thing he had ever seen in more than four decades in the neighbourhood.
A vigil held in the area on Tuesday night drew a number of mourners who gathered to remember those killed in the shooting. Some carried flowers, others held candles and many shed tears as they stood in small groups.
Mayor Rob Ford visited the scene of what he called a "horrible tragedy" earlier on Tuesday and sought to reassure residents that Toronto remains safe, despite several high-profile shootings in recent weeks.
"We've had a couple of isolated incidents ... I know this is the safest city in the world, I've always said that, I truly believe it," said Ford.
"Everyone has to move on and carry on with their life," he added.
Federal and provincial officials quickly condemned the violence.
"The actions that unfolded there sadden us and anger us at the same time," Ontario Premier Dalton McGuinty said. "They are, in a word, outrageous, something that we cannot and will not stand for, we will not tolerate."
Monday night's shooting came just weeks after a gunman opened fire in a crowded food court at Toronto's Eaton Centre. One man was killed instantly, another died of his injuries days later and several bystanders were sent to hospital.
In another recent incident, a man was shot in Toronto's east end shortly after the Canada Day fireworks display at the city's waterfront.
As well, a man was gunned down at a crowded cafe in Toronto's Little Italy on June 18.
On Tuesday night, Toronto police were investigating a shooting death in the city’s west end. A man believed to be in his 30s was shot and killed at a soccer field.
The unidentified victim was pronounced dead at the scene. He is Toronto's 29th murder victim of 2012.