CHICAGO » County prosecutors are expected to charge a white Chicago police officer with murder in the fatal shooting a black teenager, an official close to the investigation said Monday night.

The official told The Associated Press that Officer Jason Van Dyke is expected to be charged Tuesday. The official spoke on the condition of anonymity so as not to pre-empt the expected charges.

An attorney for Van Dyke did not respond to messages from the AP seeking comment.

The charge would come as the city prepares to release squad-car video of Van Dyke shooting 17-year-old Laquan McDonald 16 times on Oct. 20, 2014.

Several people who have seen the video say it shows McDonald armed with a small knife and walking away from several officers. They say Van Dyke opened fire from about 15 feet and kept shooting after the teen fell to the ground. An autopsy report says he was shot at least twice in his back.

Ministers, community leaders and others worry the images could lead to the kind of unrest seen in Baltimore and Ferguson, Missouri, after police-involved deaths. Mayor Rahm Emanuel called together a number of community leaders Monday to appeal for help in keeping the city calm.

But some said after the meeting that city officials waited too long to get them involved — more than a year after McDonald was shot.

"You had this tape for a year and you are only talking to us now because you need our help keeping things calm," one of the ministers, Corey Brooks, said after the meeting.

A judge last week ordered the Police Department to release the squad car dashcam footage by Wednesday after the city refused to do so for several months, saying the investigations into the shooting weren’t complete. The FBI and the Cook County State’s Attorney’s Office are investigating.

Ira Acree, who described the meeting with Emanuel as "very tense, very contentious," said the mayor expressed concerns about the prospect of any demonstrations getting out of control.

Another minister who attended, Jedidiah Brown, said emotions were running so high that there would be no stopping major protests once the video is released.

Earlier Monday, Emanuel’s office characterized the discussion as something "we regularly do on important topics." But Acree and another minister, Marshall Hatch, said it is a rare occurrence.

"We have been trying to meet with the mayor since the beginning of the year to talk about community relations and his staff asks for a letter and says, ‘We’ll get back to you,’ but they never do," Acree said before going to City Hall for the discussion.

Hatch added: "This has the feeling of them scrambling."

Acree and Hatch said blacks in the city are upset about the shooting and because city officials and the Police Department refused for several months to release the video until ordered to do so by a judge. They said people also are angry because the officer, though stripped of his police powers, has been assigned to desk duty and not fired.

"They had the opportunity to be a good example and a model across the country on how to improve police and community relations and they missed it," Acree said.

The Police Department said placing an officer on desk duty after a shooting is standard procedure and that it is prohibited from doing anything more during the investigations.