Laquan McDonald was walking down the middle of a Chicago street in 2014. He was carrying a knife with a 3-inch blade.

Some police officers pulled up. One of them got out of his cruiser and then shot McDonald to death. That’s not all he did. He emptied his service pistol into McDonald.

Sixteen rounds, man!

That was more than a year ago.

This week, Chicago authorities have charged former Officer Jason Van Dyke with murder in McDonald’s death.

I’ve seen the dash-cam video of the incident. It’s about 6 minutes long. Having seen it, I am compelled to join many others in asking: What on Earth took ’em so long to charge the officer with a crime?

The video is graphic. It shows McDonald, who was 17 years of age, simply walking past Van Dyke’s SUV cruiser. Remember, he wasn’t packing any firepower; he was holding a small knife.

The officer opened fire.

I must point out here — as if you need reminding — that McDonald was black and Van Slyke is white.

McDonald’s family didn’t want the video released. To its credit, the network on which I saw the video, NBC, had the decency to blur the image of McDonald lying on the ground as he was being hit by the bullets.

According to the Chicago Tribune: “Cook County State’s Attorney Anita Alvarez said she had decided weeks ago to charge Van Dyke weeks ago but was holding off until federal authorities completed their part of the joint investigation. She said she ‘moved up’ her decision to charge Van Dyke after a Cook County judge ruled last week that the video should be released to the public.”

Van Dyke had been taken off of patrol duty and was working behind a desk for the past year — while drawing his salary. He’s no longer drawing it now that he’s been charged with murder.

Well, OK. But based on what millions of Americans have now witnessed on that hideous video, it seems — to me, at least — that the “joint investigation” could have been wrapped up months ago.

What took ’em  so long?