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

A police officer has been charged with murder for the death of Laquan McDonald, a black teenager who was shot 16 times after being stopped by officers on a Chicago street.

The state prosecutor has described the shooting as "chilling" and warned that the video will "tear at the hearts" of people in Chicago when it is made public on Wednesday.

She set out the state's case against the officer, Jason Van Dyke, and what she has ascertained about his actions that night in 2014.

Mr Van Dyke maintains he feared for his life when he opened fire, because the teenager was armed with a knife.

Here is what prosecutors say happened, in numbers.

McDonald was holding a knife with a three-inch blade when police spotted him and pulled over. Officer Van Dyke was not among the first officers to attend.

The teenager had slashed the tyre on one of the patrol cars and there was a stand-off between him and the officers.

There were no words spoken by McDonald during the incident. He said nothing in response to numerous verbal commands to drop the knife, police said.

The first responding officer said he did not see the need to use force.

But an officer armed with a Taser was requested to attend and deal with the situation but none arrived.

Officer Van Dyke, who arrived in a patrol car with a partner, was at the scene for less than 30 seconds before he started shooting.

He was out of his car for approximately six seconds before he opened fire.

According to police dashcam video, the time was exactly 9:57:36 on the October 2014 night when the officer opened fire.

There were eight officers present but only one fired his or her weapon.

Officer Van Dyke's gun was a nine-millimetre calibre semi-automatic pistol with a capacity of 16 rounds.

Officer Van Dyke shot the teenager 16 times. The 16 fired cartridge cases police recovered at the scene were all from his weapon.

About 14-15 seconds elapsed between the time the first shot was fired and the last. An eyewitness told police he heard a pause after the initial shots, and then more gunfire. The prosecutor says the video shows how McDonald's body jerked and his arm moved slightly.

McDonald was lying on the ground for 13 of these seconds as the shots were fired. Officer Van Dyke was preparing to reload when his partner, who could hear McDonald struggling to breathe, told him to hold his fire.

The prosecutor said the teenager still had a pulse when paramedics arrived but was declared dead at the hospital.