Experts cannot agree on what to call a recent rise in homicides, much less its cause, but new data on Friday that showed a sharp spike in homicide rates in more than 20 cities rekindled debate over whether it was time for alarm.

The data showed particularly significant increases in homicides in six cities in the first three months of the year compared with the same period last year — Chicago, Dallas, Jacksonville, Fla., Las Vegas, Los Angeles and Memphis. But almost as many cities reported a notable decline in recent months.

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