When President Trump talks about sending the feds to Chicago to work on the city’s uniquely violent culture and apparently intractable murder problem, hopefully he won’t just unthinkingly send troops and tanks to occupy the West Side at night.

An insightful piece at Bloomberg from last week offers some hints about at least one way the feds actually could help reduce murder rates in the handful of cities where they have spiked over the last few years: Make it easier for beleaguered police departments to share and use data in finding serial killers — the ones whose killings tend to form patterns in the data, if anyone bothers to look closely. Unfortunately, many police departments don’t do this at all.