A new report on gun violence in the United States highlights the connection between distrust with the police to lower crime reporting and increased vigilante justice.

The report by the Giffords Law Center to Prevent Gun Violence explores a spike in gun violence in many major American cities between 2014 and 2017 and its connection to police violence and growing distrust between police and the communities they serve.

"This report condenses the leading recent research in the field to explain how cycles of distrust and disengagement fuel cycles of violence," the report's executive summary said.

That wariness leads victims to not report crimes or participate as witnesses in investigations, the report concluded. After high-profile instances of police brutality, 911 calls to law enforcement drop in part of a "cycle of distrust" that makes it harder for police to solve crimes, thus making arrest numbers decline.

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"Grieving witnesses and survivors need to be able to rely on their police forces to keep them safe, treat them fairly, and respond to their communities' priorities, or else a desperate few will resort to vigilante violence instead," Ari Freilich, state policy director for the Giffords Law Center, said in a statement.

"But the simple truth is that for millions of Americans, especially communities of colour, the status quo is failing to earn that trust or deliver safety or justice."

One example cited by the report was a 2016 study by Harvard, Yale, and Oxford researchers, which detailed how the Milwaukee beating of 26-year-old Frank Jude by a group of off-duty police officers outside of a party in 2004 lead to a 20 percent drop in emergency calls reporting crimes in the city, despite an increase in violence during the same period.

The researchers found that the number of calls from white neighbourhoods briefly dropped after the incident became public, but soon returned to normal, while the drop in predominantly black neighbourhoods was "large and durable" for more than a year.

The Giffords Law Center report, which explores how gun violence is connected to racial inequality, also highlights several cities, including Camden, New Jersey, Stockton, California, and Minneapolis, Minnesota, where reform has proven effective.

Jesse Jannetta, a senior policy fellow at the Urban Institute whose work is cited by the report, told Al Jazeera the findings highlight "emerging thinking" on how police forces should prioritise trust-building.

"Particularly when you've got a community where levels of violence are high, [trust-building] is rarely one of the first things named in work that needs to be done," said Jannetta. "Often, a lot of the immediate response that comes when there are spikes in violence is a lot of heavy enforcement which, in fact, can exacerbate trust issues where they already exist, and make it harder to build peace and maintain it over a long term."

Highlights on race and violence

Violence continues to disproportionately affect young men in communities of colour, the report said, with violence accounting for 50 percent of the deaths of young black men and boys aged 15 to 24 in the US in 2016 and 20 percent of the deaths of Hispanic men and boys in the same age range during that year. Violence only accounted for 4 percent of deaths of white men and boys aged 15 to 24.

Overall, African American citizens accounted for 51 percent of gun homicide victims nationally from 2010 to 2017, while only accounting for 7 percent of the US population, the report said.

Furthermore, gun violence in the US continues to happen in concentrated areas. In 2015, more than a quarter of gun homicides happened in city neighbourhoods accounting for just 1.5 percent of the country's entire population.

Nationally, nearly one-third of Americans seriously injured in crimes involving weapons did not report the crime to authorities, the report says, while more than half of homicides of African Americans never led to an arrest, according to a Washington Post investigation cited by the report.

"When the formal justice system is seen as absent, abusive, or ineffective, a small number of individuals are compelled toward violent vigilantism instead," says the report, which cites studies showing that victims of violence are more likely to later perpetrate violence and join street cliques for self-defence.

Because only a small fraction - on average about 0.6 percent of a city's population - are responsible for homicides and non-fatal shootings, the report concluded that police strategies are needed that "treat the remaining 99 percent of the population as victims, witnesses, and critical partners in addressing violent crime, instead of part of the problem".