Cover-up: How Rahm Emanuel won the mayor's race by suppressing the Laquan McDonald video

Then up for re-election, the mayor of Chicago, Rahm Emanuel, was looking ahead to a contested election on Feb. 24, 2015, which would ultimately result in a runoff election on April 7. In Ferguson, Mo., a grand jury was hearing testimony on the police shooting of Michael Brown. The video of Eric Garner being choked to death during an arrest in New York had gone viral. The Black Lives Matter movement was gaining momentum across the country.

The video of a police shooting like this in Chicago could have buried Mr. Emanuel’s chances for re-election. And it would likely have ended the career of the police superintendent, Garry F. McCarthy.

And so the wheels of justice virtually ground to a halt. Mayor Emanuel refused to make the dash-cam video public, going to court to prevent its release. The city argued that releasing the video would taint the investigation of the case, but even the attorney general of Illinois urged the city to make it available.

Then the city waited until April 15 — one week after Mr. Emanuel was re-elected — to get final approval of a pre-emptive $5 million settlement with Mr. McDonald’s family, a settlement that had been substantially agreed upon weeks earlier. Still, the city’s lawyers made sure to include a clause that kept the dash-cam video confidential.