After a year in which murders spiked in the U.S., homicides are already trending up in many cities, presaging whatis likely to be a violent summer.

Why it matters: The rise in homicides is a public health crisis that has multiple interlocking causes, which makes solving it that much more difficult. We're still a long way from the murderous days of the 1990s, but rising gun violence is destroying lives and complicating efforts to help cities recover from COVID-19.

Driving the news: From Washington to Louisville, Kentucky, New York to Oakland, California, and Kansas City to Atlanta, murder rates are trending up in U.S. cities large and small.

• A sample of 37 cities with data available for the first three months of 2021 collected by the crime analyst Jeff Asher indicates murders are up 18% over the same period in 2020.
• The continued increase comes after a year in which major U.S. cities experienced a 33% rise in homicides, and 63 of the 66 largest police jurisdictions saw an increase in at least one category of violent crime, according to a report from the Major Cities Chiefs Association.

Between the lines: While it may be tempting to dismiss 2020 and the early indicators in 2021 as aberrations caused by the pandemic, murder rates were already ticking upward in the years before the pandemic.

• After hitting a modern-day low in 2014 following a quarter-century of general decline, homicide rates began rising again in many cities.

The intrigue: Criminologists still haven't settled on a single explanation for why violent crime dropped drastically from the 1990s, and they're even less certain why it's risen so dramatically over the past 16 months.

• The direct and indirect effects of the pandemic almost certainly play a major role, with in-person schools closed, violence prevention programs forced to pull back and unemployment skyrocketing, especially in big cities.
• But property crimes like robberies mostly continued falling, and historically there's no clear link between periods of economic disruption and murder rates.
• While some police leaders blame resourcing issues because of the nationwide marches that followed the killing of George Floyd by police last summer, with a few exceptions there's little evidence the protests directly led to a rise in the murder rate. But a general pulling back of policing — plus the distancing effects of the pandemic itself and the closure of courts — likely contributed to more murders and fewer of them being solved.
• 2020 saw a historic increase in firearm purchases — especially among first-time buyers — adding to a nationwide arsenal that was already overflowing with guns. More guns at the ready mean disputes can rise more quickly to the level of shootings, and lack of trust in the police makes people more willing to take matters into their own hands.

Yes, but: Even if 2021 eclipses last year's murder numbers, America will remain a far safer country than it was during the most violent years of the 1990s.

• New York City recorded 462 homicides in 2020, an increase of nearly 45%, but in 1990 the city recorded 2,605 murders — more than seven per day.

What's next: A violent summer on America's streets appears likely, given that homicide already appears to be trending above last year's spike.

• Homicide rates historically spike during the summer months, when the hotter weather puts more people on the streets, and while vaccination coverage is increasing, the pandemic and all its knock-on effects won't be finished by then.
• "Summer 2021 is going to be abnormally violent," John Roman, a senior fellow at the economics, justice and society group at NORC at the University of Chicago, wrote this year. "It is the new normal."

The bottom line: The historic decline in murder over the past few decades was accompanied by mass incarcerations and increasingly brutal policing, leading to what the criminologist Patrick Sharkey termed "the uneasy peace."

• As America reckons a new murder wave with policing in the post-George Floyd killing era, it needs to find a way to a lasting peace that features both safety and justice.