A shooting at a packed nightclub, another outside a North York baby shower, a downtown drive-by in broad daylight. If Torontonians remember 2015 as a year of brazen gun crimes, it won't be their imaginations. Shootings spiked – though not shooting deaths – compared to the past four years.

As police reports mounted, so did theories about what was to blame. Criminologists, the police chief and those who follow gang activity are still puzzling over what sparked the spate of violence, everything from a leaky border to the weather – and whether those problems were unique to 2015 or part of a trend that will continue in the new year.

After one particularly bloody weekend in December, Toronto Councillor Giorgio Mammoliti even advocated calling in the army to crack down on what he called a new gang "turf war."

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But those who work with young people at risk of gang recruitment say that, while retaliation is a factor, the theory of a bigger inter-gang war doesn't ring true with what they're hearing.

Gang shakeups have "a presence – you feel it," said Steven Ambrose, a basketball coach who works with teenagers in the Lawrence Heights and Jane-and-Finch areas, and often hears about gang activity.

"If it was simply due to a turf war, I think the streets would be hotter," he said. "People would be talking about it more, internally, among themselves, but I haven't heard any of that chatter, so I've got to assume it's just normal – guns coming across the border."

The border took the blame after several shootings, including the July death of 14-year-old Rexdale girl Lecent Ross, who was shot in the head in a room full of teenagers.

Nigel Barriffe, a teacher at Lecent's school, said the flow of illegal guns from the United States was making it "just too easy for guns to be getting into all communities" and into the hands of young people.

Border problems can't be ignored, said Mark Pugash, spokesman for Toronto Police Services.

"We have worked with the [Canada Border Services Agency] and others for many years, and it's a very significant challenge," he said. "That's an important piece."

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But is there really a new deluge of firearms from the south? Detective Inspector Peter Goodwin of the Ontario Provincial Police told the Hamilton Spectator in August that the U.S. is still the biggest source of illegal guns in Ontario, but that co-operation between Canadian and U.S. law enforcement seems to be making a difference. While it was once common to uncover large shipments of guns at the border, now they seem to be coming across in small packages of two or three, he said.

A growing problem in Ontario is domestic weapons that are stolen and then trafficked within the province, said Det. Insp. Goodwin. In many of the thefts, legal gun-owners are followed from gun shops or shooting ranges.

However, many people agreed that wherever they're from, guns are more available than ever to young people – and that Toronto criminals are intentionally recruiting very young teens and sometimes asking them to act as "mules" to carry the firearms.

Criminologists and sociologists always keep in mind that a spike in crime can sometimes mean simply that people are reporting it more often, said Jooyoung Lee, a sociology professor at the University of Toronto who studies local gun crime. Reporting can improve when police make an effort to build trust – or when new fears prompt people to turn to police.

"It could just be that something else is happening in these neighbourhoods where there's a culture emerging, where people feel helpless," Mr. Lee said.

In a year-end interview with CTV, Toronto Police Chief Mark Saunders also tied the change to police practices, using different logic. He said the spike is a "complex question," but he believes criminals have been emboldened by the curtailment of the force's carding policy, which allowed officers to approach people and gather their personal information even when they weren't suspected of a crime.

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"Informants, wiretapped information and agents have told us that the criminal elements understand that police are not approaching them, so they feel safer carrying firearms," the chief said.

There are simpler explanations, starting with the woes of the Canadian economy, Mr. Lee said.

"One hypothesis could be that a downturn in the larger economy could also cause a spike in gun-violence rates," he said. "Canada is in a little bit of an economic rut."

Add to that the unseasonably balmy final months of 2015, and the fact that crime is known to increase with warm weather, Mr. Lee said.

Other kinds of violent crime also jumped this year, including a total of 17 stabbing deaths, the highest number since 2011, the most recent year for which police publicize statistics.

But before jumping to conclusions about statistics, it's important to remember that Toronto is growing quickly right now, and that recent years' numbers may not have been adjusted against population size, Mr. Lee said.

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"On paper it sort of produces this sense of fear and anxiety that there's a crime wave or that things are out of control, but … it's hard to get a read on what's happening," he said.

Mr. Pugash said a police working group is analyzing what happened this year, but so far none of its conclusions have been made public.

"I think a lot of people want a simple solution to a difficult issue," he said. "If you talk to people who actually study this for a living … there is virtually never just a single factor or two that plays a significant role."