For a moment at least, the biggest political beneficiary of the shootings in San Bernardino wasn’t a presidential candidate or terrorist group. It was Chicago Mayor Rahm Emanuel. The killing spree in Southern California distracted the national press at precisely the moment it was about to go into a feeding frenzy over a scandal consuming his mayoralty.

If such political scorekeeping seems gauche, it’s a fitting tribute to Emanuel, who may have participated in a political cover-up.

Last year, police officer Jason Van Dyke killed Laquan McDonald, a black teenager. Van Dyke put 16 bullets in him. Five other officers claimed McDonald had lunged at them menacingly with a knife.

But that’s not what happened. A police dash-cam video shows that although McDonald was carrying a knife, he was walking away from Van Dyke when the cop fired the first bullet. McDonald was lying on the pavement, motionless, for the next 15. The officer was out of his car for just seconds before he started shooting.

Perhaps reasonable people — or at least Van Dyke’s lawyers — can disagree about the first bullet, though I can’t see what that argument would be. But the next 15?

The homicide occurred in October 2014. Emanuel was facing a tough primary fight in a city where the Democratic primary is the only election that matters. The furor over the Michael Brown shooting in Ferguson, Mo. — which birthed the Black Lives Matter movement — was in full steam thanks to widespread coverage of the grand jury looking into the case. If the McDonald video had come out before the primary or the subsequent runoff, Emanuel would have been toast.

Thanks to stonewalling by the Emanuel administration and his handpicked chief of police, the video didn’t surface for more than a year. The video might not have surfaced at all if it hadn’t been for a court-ordered release at the request of a very persistent journalist. And murder charges came only after it became clear the journalist would get his way.

There is more evidence of a cover-up. Remember that five police officers reported events contradicted by the video. Presumably relevant security footage from a nearby Burger King was allegedly erased by police. The McDonald family was given a $5 million settlement. At a press conference Monday, Emanuel said he “welcomes” a federal investigation of the killing — but what else could he have said?

Normally, the rule in politics is that the cover-up is worse than the crime. It’s hard to make that case here. But that doesn’t mean the apparent conspiracy to hide the crime is trivial.

This scandal should be instructive. Conservatives, including me, have generally been dismissive or even contemptuous of the Black Lives Matter movement — and the movement has done much to earn that scorn. Michael Brown was not murdered. The whole “Hands up, don’t shoot” narrative was based on deliberate mythmaking. And the requirement that Democratic politicians be barred from uttering the phrase “All lives matter” is an absurdity out of a Tom Wolfe novel.

But the Chicago scandal isn’t that. Whereas the evidence didn’t support the protesters in the case of Michael Brown, it certainly appears to in the case of Laquan McDonald. Moreover, the Chicago Police Department has an ugly past, and this situation suggests it has an ugly present. Conservatives should pause and consider that despite all of the posturing and hysteria elsewhere, there’s real substance to the movement’s complaints in the Windy City.

The McDonald killing should also spur some reflection among liberals. There’s no simple political pattern to where police abuses arise; they’re certainly not confined to red states, or absent in blue states. But it is easier to predict where politicians will get away with covering them up: cities suffering from one-party rule.

You know the phrase “the best thing since sliced bread”? It’s used to convey that something hasn’t happened in a long time. Sliced bread went on the market in 1928. That was one year after Chicago last elected a Republican mayor.

Monopolistic political machines breed contempt for voters, animosity toward policy innovation and even minimal notions of transparency. Ending one-party rule in Chicago and other big cities won’t solve all of their problems, but it can force a more honest conversation about what those problems are.

© 2015 TRIBUNE CONTENT AGENCY, LLC