CHICAGO

- A white Chicago police officer who shot a black teenager 16 times last year was charged with first-degree murder yesterday, hours before the city released a video of the killing that many people fear could spark unrest.

City officials and community leaders have been bracing for the release of the dash-cam video, fearing the kind of turmoil that occurred in cities such as Baltimore and Ferguson, Mo., after young black men were slain by police or died in police custody.

A judge ordered that the recording be made public by today. Moments before the footage was released last night, Mayor Rahm Emanuel and Police Chief Garry McCarthy appealed for calm.

The relevant portion of the video runs for about 40 seconds and has no audio.

Laquan McDonald, 17, swings into view on a four-lane street where police vehicles are stopped in the middle of the roadway. As he jogs down the street, he appears to pull up his pants and then slows to a brisk walk, veering away from two officers who are emerging from a vehicle and drawing their guns.

Almost immediately, one of the officers appears to fire from close range. McDonald spins around and crumples to the pavement. The second officer simultaneously lowers his weapon.

The car with the camera continues to roll forward until the officers are out of the frame. Then McDonald can be seen lying on the ground, moving occasionally. At least two small puffs of smoke are seen coming off his body as the officer continues to fire.

In the final moments, an officer kicks something out of McDonald's hands.

Police have said the teen had a knife. Cook County State's Attorney Anita Alvarez said yesterday that a 3-inch knife with its blade folded into the handle was recovered from the scene.

Shortly after the video's release, protesters began to march through streets. Several hundred people blocked traffic on the near West Side. Some circled police cars in an intersection and chanted "16 shots." No violence was immediately reported.

City officials spent months arguing that the footage could not be made public until the conclusion of several investigations. After the judge's order, the investigations were quickly wrapped up and a charge announced.

Alvarez defended the 13 months it took to charge Officer Jason Van Dyke. She said that cases involving police present "highly complex" legal issues and that she would rather take the time to get it right. Alvarez said concern about the impending release prompted her to move up the announcement of the murder charge.

"It is graphic. It is violent. It is chilling," she said. "To watch a 17-year-old young man die in such a violent manner is deeply disturbing. I have absolutely no doubt that this video will tear at the hearts of all Chicagoans."

But she insisted that she made a decision "weeks ago" to charge Van Dyke and the video's ordered release did not influence that.

Some community leaders said there was no doubt that Alvarez only brought charges because of the order to release the video from Oct. 20, 2014.

"This is a panicky reaction to an institutional crisis within the criminal-justice system," said the Rev. Jesse Jackson, who said he hoped to see "massive" but peaceful demonstrations.