The clash at the July 16 block party on Danzig Street left Shyanne Charles, 14, and Joshua Yasay, 23 — who police have said were innocent partygoers — dead and more than 20 others wounded.
Members of the Galloway Boys, a local street gang, were at the barbecue and "took ownership" of it, said Staff Insp. Greg McLane.
A member of another gang, the Malvern Crew, showed up at the party but was turned away, McLane said.
"It's alleged that the Malvern Crew member left the barbecue, called a number of his associate gang members and attended back at the barbecue to confront the members of the Galloway Boys to seek revenge for being turned away," he said at a news conference.
"This resulted in a violent confrontation between a number of gang members from the Galloway Boys and from the Malvern Crew that involved a number of firearms."
Police announced Thursday that Shaquan "Bam Bam" Mesquito, 18, has been charged with two counts of first-degree murder, attempted murder, reckless discharge of a firearm and 23 counts of aggravated assault.
The first murder charges in the shooting are a milestone in the investigation, said Toronto Police Chief Bill Blair, but he called more arrests "inevitable."
"There are others who also bear a responsibility for what took place in the Danzig community on July 16 and we're going to hold them to account," he said.
Mesquito faces two counts of first-degree murder, but it doesn't mean police necessarily allege he fired the shots that killed Charles and Yasay.
"I'm not saying that he's solely responsible for all the carnage that happened that day," McLane said.
Police likened the scenario to a shootout on Boxing Day 2005 in Toronto that killed 15-year-old Jane Creba.
Gunfire erupted as two rival groups of young men confronted each other on Yonge Street. Police were unable to determine whose gun actually fired the shot that killed Creba, but several people were convicted of murder and manslaughter charges.
The courts held that all those convicted in Creba's case were participating in a reckless activity likely to injure or kill someone and therefore bear responsibility for her death.
Mesquito has already been in jail since shortly after the shooting, facing a charge of threatening to cause serious bodily harm also related to the barbecue. He appeared in court Wednesday on the new charges and was remanded into custody.
Police also released composite sketches of two people of interest. They are both described as black men in their 20s, one with a scar under his left eye. McLane said it's not yet known if they were involved or were just witnesses, but police would like to speak with them.
Another suspect in the incident, Nahom Tsegazab, 19, was charged earlier this year with reckless discharge of a firearm.
Police have said the Galloway Boys have been linked to at least five other shootings in the past year and they are aware of conflicts between them and the Malvern Crew.
Blair described the gangs' strength as "significantly diminished," but McLane noted that others wait in the wings, ready to take up the mantle.
"My understanding is that the Malvern Crew and the Galloway Boys are in a period of transition because of some significant arrests that were made historically," he said.
"There are people that will always try to take up the void in these gangs and make themselves known to other members of the community."
Blair said he understands community members' fear of coming forward to police with information and tried to provide assurances that police will deal with their tips sensitively.
"We will provide all of the support to individuals who do come forward because we need their help and they need our support," he said.
"We need people to step forward and tell us what they know."
Note to readers: This is a corrected story. Information initially provided by police said Mesquito faced 24 charges of aggravated assault.
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