There is a whole arsenal of options available to the President.
These are just a few... - Increase FBI and DOJ resources for ongoing cases; prioritizing homicide and firearm related cases. - Exert Federal jurisdiction on current CPD investigations and in-flight prosecutions where possible, move to Federal courts, Federal sentences. - Increase US Marshal Service resources for fugitive apprehension and serving Federal warrants, prioritize homicide and firearms related cases. - Open a DOJ civil rights investigation into the City of Chicago; i.e. is the crime and homicide rates becoming a civil rights issue for minority areas. Are current leftist policies regarding the police contributing to increased crime. - Open a DOJ investigation into whether Chicago's sanctuary city policies are in violation of Federal law. - Increase ATF resources to help address the firearms issues. - Increase ICE resources and prioritize Chicago for high risk offenders. - Offer federal funding for additional police resources. -etc.
Essentially the feds can crawl up Chicago's ass until they either fix this themselves, or the feds do it for them.