WASHINGTON (AP) — President Donald Trump is serving notice he’s ready to “send in the Feds” if Chicago can’t reduce its homicide figures.

Trump singled out Chicago’s gun violence in a tweet Tuesday night.

If Chicago doesn’t fix the horrible “carnage” going on, 228 shootings in 2017 with 42 killings (up 24% from 2016), I will send in the Feds!

— Donald J. Trump (@realDonaldTrump) January 25, 2017

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Chicago Police Superintendent Eddie Johnson responded late Tuesday, saying: “The Chicago Police Department is more than willing to work with the federal government to build on our partnerships with DOJ (Department of Justice), FBI, DEA (Drug Enforcement Administration) and ATF (Bureau of Alcohol, Tobacco and Firearms) and boost federal prosecution rates for gun crimes in Chicago.”

Mayor Rahm Emanuel criticized Trump on Monday for worrying about the size of the crowd at his inauguration. Emanuel, a longtime political ally of former President Barack Obama, also acknowledged his own frustration with Chicago’s crime rate.

Earlier this month, before he took office, Trump tweeted that Emanuel should ask for federal help if he isn’t able to bring down the homicide rate. Last year, the death toll soared to 762 — the most killings in the city in nearly two decades and more than New York and Los Angeles combined.

This year’s numbers cited by Trump were slightly different to the latest tally from the Chicago Police Department. As of Tuesday, Chicago police said 234 people have been shot in 2017, including 38 who died. At this point last year, according to Chicago police, there had been 227 shot in 2016, including 33 deaths.

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Trump isn’t the first to broach the idea that the U.S. government could do more to help stem predominantly gang-related violence on Chicago’s South and West sides. But now, as on earlier occasions, what more the federal government can do isn’t at all clear.

Here’s a look at the issue and some of the options that might be available:

National Guard: The most direct — and most extreme — intervention would be sending National Guard troops into Chicago to try and tamp the violence. Both the U.S. president and the Illinois governor have the authority to mobilize the Illinois National Guard. But a federal statute, known as the Posse Comitatus Act of 1878, prohibits the deployment of federal troops in civil law enforcement. While a president could try to find legal loopholes to get around that prohibition, the legalities of any such deployment would pose major obstacles. A heavy-handed intervention could upset many conservatives who advocate for states’ rights.

Trump has not broached the possibility of using troops, although the idea comes up from time to time in Illinois. When someone asked Illinois Gov. Bruce Rauner about the possibility in August, he ruled it out. He said he had discussed the idea with police, community leaders and the National Guard and that “no thoughtful leader thinks that’s a good idea or (that it would) would really provide a solution.”

U.S. Attorney: Current U.S. Attorney Zachary Fardon came under pressure when selected to be the top federal law enforcement official in Chicago three years ago to put violent crime at the top of the city’s agenda. But from the start of his tenure, he tempered expectations about what a U.S. attorney’s office could do. He had said repeatedly that communities couldn’t arrest their way out of the problem of violence and that a more holistic approach — one that also addresses socio-economic problems — is required.

Fardon said in a September speech that his office and federal agencies including the FBI, DEA and Bureau of Alcohol, Tobacco, Firearms and Explosives “have ramped up their resources to help tamp down this spike in violence.” He also said federal agencies had launched new crime-fighting programs with city and state officials, though he didn’t provide details. “Suffice it to say, all oars from those agencies, including the FBI, are in the water right now,” Fardon said.

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Gun Charges: Then-U.S. Attorney General Eric Holder promised in 2014 that the Department of Justice would do what it could to fight gun violence in Chicago. That was easier said than done. A records review by the Chicago Sun-Times found numbers of federal gun cases out of Chicago declined from 2012 to 2015. It started trending back up in 2015, with 84 defendants charged under U.S. gun statutes from mid-2015 and mid-2016. Responding to the Sun-Times, Fardon wrote in October that the drop-off in gun charges in 2014 to less than 60 had at least something to do with a federal hiring freeze, when his office went from 172 prosecutors to below 130. The numbers are getting back up to where they need to be, he said. He said half of his prosecutors are focused on the types of cases linked to street-level violence.

Gangs: Going back decades, a cornerstone of anti-violence measures by U.S. attorneys in Chicago has been to convict and imprison gang leaders. Federal prosecutors in Chicago have used racketeering and gun laws to go after gangs, who police blame for much of the deadly violence. One of the largest street-gang cases in recent Chicago history wrapped up this month, with jurors convicting the core leadership of the notorious Hobos street gang on trial for an alleged racketeering conspiracy that prosecutors say included at least nine killings. But many gang experts point to an unintended consequence of prosecuting gang leaders: Breaking up a gang’s command structure can often lead to more inter-gang rivalry and, therefore, to even more violence.

Trump isn’t offering specifics about how the federal government could help. 

The president frequently mentioned Chicago’s gun violence during the campaign trail and has vowed to reduce crime. The recently revamped White House website mentions Chicago’s gun violence in a section called “Standing Up For Our Law Enforcement Community.” In it, the White House says, “There were thousands of shootings in Chicago last year alone. Our country needs more law enforcement, more community engagement, and more effective policing.”

Trump’s tweet did not go over with some aldermen, who gathered Wednesday for a monthly City Council meeting. 

Alderman @JohnArena445 on Trump tweet: “He can make sure the DOJ works with us to create a consent decree that we can enforce”

— Lauren Chooljian (@laurenchooljian) January 25, 2017