Image copyright AP Image caption The officer who shot the black teenager turned himself in

A white Chicago police officer who shot a black teenager 16 times last year has been charged with first-degree murder, state lawyers have said.

Jason Van Dyke was not justified in shooting 17-year-old Laquan McDonald, state lawyer Anita Alvarez said.

A police dashcam video to be released on Wednesday is "chilling", she added, and will "tear at the hearts".

Police use of excessive force against African Americans has been the subject of nationwide protests for months.

In Minneapolis on Monday night, one of those protests was marred by violence when three white men opened fire at a rally, injuring five people.

In the Chicago case, a post-mortem showed that McDonald was shot at least twice in the back.

Step-by-step guide to what happened

Ms Alvarez said McDonald was carrying a knife when he was stopped by police in 2014 but was not threatening them or moving towards them.

Mr Van Dyke opened fire seconds after he arrived and continued shooting at the teenager as he lay on the ground, she said.

Image copyright Chicago Tribune Image caption The video of McDonald's shooting will be made public

"While we expect officers to do their jobs... there are a few bad apples who go too far and break the law," she said.

Chicago Mayor Rahm Emanuel has said the officer was not justified in killing the teenager, calling his actions "hideous".

"At every point, he violated what we entrust," Mr Emanuel said on Monday.

Illinois Governor Bruce Rauner has said the video is "very troubling" and he expects a strong public reaction, though he hopes it will be "thoughtful and peaceful".

The police union in Chicago has opposed the release of the video and said it would taint a trial jury.

The officer, through his lawyer and the the police union, has said he feared for his life because McDonald was holding a knife.

Chicago police have also said McDonald threatened officers with a knife and slashed the tyres and windows of a patrol car.