CHICAGO — President Donald Trump is again warning Chicago about its high number of homicides, saying on Twitter that he is ready to “send in the Feds” if the nation’s third largest city can’t reduce the number of killings on its own.

Trump has offered no specifics on what kind of federal intervention he envisions, but many steps he could potentially contemplate pose practical and even constitutional obstacles.

Mayor Rahm Emanuel said Tuesday that additional federal intervention could include funds to help hire more police officers.

Here’s a look at some of the issues:

NATIONAL GUARD

A federal statute, the Posse Comitatus Act of 1878, prohibits the deployment of federal troops to do the jobs of domestic police. While a president could seek to use narrow, rarely invoked used exceptions in that statute, civil libertarians would likely protest any such move.

Sending armed troops to stand on neighborhood street corners could also undermine what city of Chicago officials have said is a key to improving policing on Chicago’s South and West Sides, where most homicides occur: Building trust between law enforcement and residents in those minority communities. Emanuel said Wednesday sending in guardsmen would be “antithetical” to trust-building.

JUSTICE DEPARTMENT

A damning report released just before Trump’s inauguration cited excessive use of police force and other problems that not only violated constitutional rights but hampered crime-fighting. Such findings are supposed to trigger a process that, under Obama, typically led to police-reform plans overseen by a U.S. judge. Some activists worry that Trump — endorsed by Chicago’s police union during the campaign — won’t apply as much pressure on Chicago police to change.

THE MAYOR

Among the list of things Mayor Emanuel told WTTW’s “Chicago Tonight” the federal government could do was to devote even more resources to tracking the flow of illegal guns into Chicago from neighboring Indiana, where gun laws aren’t as strict as in Illinois; as well as putting more money into federal after-school programs and “investing in neighborhoods that are hard hit by poverty (that) become a breeding ground for violence.”

GANGS

Federal prosecutors in Chicago have used racketeering and gun laws to go after gangs, who police blame for much of Chicago’s deadly violence. But breaking up a gang’s command structure can often lead to more inter-gang rivalry and, therefore, to even more violence.