A day after President Donald Trump declared he was ready to “send in the Feds” if Chicago can’t reduce its homicides, Mayor Rahm Emanuel warned against deploying the National Guard, saying it would hurt efforts to restore trust in the police.

Trump offered no details on what kind of federal intervention he was suggesting or if it could involve troops, but the mayor cautioned that using the military could make matters worse.

“We’re going through a process of reinvigorating community policing, building trust between the community and law enforcement,” the mayor told reporters Wednesday. Sending troops “is antithetical to the spirit of community policing.”

He said he welcomed federal help battling “gangs, guns and drugs.”

On Tuesday night, Trump tweeted: “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!”

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

If the president was suggesting the use of federal troops, such a plan could face practical and constitutional obstacles. A law dating back to 1878 prohibits the deployment of federal troops to do the jobs of domestic police, with some rarely invoked exceptions.

In his campaign, Trump talked regularly about getting tough on crime, sometimes singling out Chicago, which was in the midst of a year in which 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.

His tweet also came less than two weeks after the Justice Department issued a scathing report that found years of civil rights violations by Chicago police. The investigation was launched after the release of a video showing the 2014 death of a black teenager who was shot 16 times by a white officer.

The Justice Department documented cases in which officers shot people who did not pose a threat and used stun guns for no other reason except that people refused officers’ commands.

Emanuel, a Democrat who once worked as former President Barack Obama’s White House chief of staff, said the police department already partners with federal agencies such as the FBI and the Drug Enforcement Administration to combat crime, including efforts to halt the flow of illegal guns pouring into Chicago from elsewhere. He said he would like to see that cooperation “expanded dramatically.”

Story by The Associated Press