There was a law passed post-Rodney King to allow the Federal Government to take over police forces that are behaving badly on civil rights. (King's civil rights weren't violated, according to me and many conservative analysts, but the first Bush Administration got convictions on some of the cops involved.)
You can read about it in Heather Mac Donald's The War On Cops. (The law is called Violent Crime Control and Law Enforcement Act (VCCLEA).)
During the Clinton Administration, and while Hillary was running for Senator from New York, the same tactics were used against Rudy Giuliani's New York. The (Bill) Clinton Justice Department investigated the cops who'd been exonerated in the Amadou Diallo shooting, on behalf of (Hillary) Clinton senatorial campaign.
Well, you know, it doesn't look as if they're trying too hard. Some days it doesn't look as if they're trying at all. (See In 2016 Chicago, Only 4% of Attempted Murderers Have Been Arrested.)
Now Trump is suggesting that instead of the Justice Department backing the criminals, they should do something about crime in Chicago.
What a difference an election makes.