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.
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
What a difference an election makes.