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Chicago saw an unwelcome uptick in violence in 2016, notching more homicides than New York City and Los Angeles combined. The gun violence is concentrated in the city's west and south sides, and most of those killed and killing are members of gangs. It is a serious problem that Chicago has been trying to address. President Trump noted on Twitter Tuesday night that the violence hasn't eased up this month, and he offered a solution, or more precisely an ultimatum:

About an hour before Trump's tweet, Fox News host Bill O'Reilly had asked on air, "Will President Trump step in and override local authorities" in Chicago as gang violence worsens?

A spokesman for Chicago Mayor Rahm Emanuel, former President Obama's first White House chief of staff, said Tuesday night that "the mayor has addressed how the federal government can help address violence several times." Emanuel had criticized Trump on Monday for fixating on the size of his inaugural crowd. Chicago Police Superintendent Eddie Johnson also responded to Trump's tweet, saying in a statement: "As the mayor said just a few hours ago, the Chicago Police Department is more than willing to work with the federal government to build on our partnerships with DOJ, FBI, DEA, and ATF, and boost federal prosecution rates for gun crimes in Chicago."

Under the Posse Comitatus Act of 1878, Trump can't deploy the National Guard for civil law enforcement in Chicago, and in August, Gov. Bruce Rauner (R) had ruled out requesting federal troops. After speaking with Chicago police and local leaders, he said, "no thoughtful leader thinks that's a good idea or would really provide a solution." Earlier this month, The Associated Press ran down what options Trump does have when it comes to Chicago violence.