More people were murdered in large U.S. cities last year than in 2014 — the first substantial increase in homicides in a quarter-century, after years of improving safety on American streets — and criminologists still are not sure why. There has been a fierce debate about the causes of the violence, but one possible explanation has not received enough public attention so far, according to the the author of a report published Wednesday by the Justice Department.

The theory — one of several that criminologist Richard Rosenfeld presents in the paper — suggests that, after a number of widely discussed law-enforcement killings of young black men during the past couple of years, residents of predominately black and disadvantaged urban neighborhoods further lost confidence in the police, the Washington Post reports.

A loss of trust could have made residents of those places less likely to share information with law enforcement about dangerous criminals. With a newfound sense of impunity, these criminals might have begun committing even more crimes. And threatened by the violence, neighbors might have armed themselves instead of going to the police for protection, the theory suggests.

"When persons do not trust the police to act on their behalf and to treat them fairly and with respect, they ... become more likely to take matters into their own hands," Rosenfeld writes. "Disputes are settled informally and often violently."