Illinois' attorney general asked Chicago police on Wednesday to release a video that allegedly shows an officer shooting a black teenager 16 times last year.

The request came a day before a judge was expected to decide whether to order police to release dashboard-camera video of the fatal shooting of 17-year-old Laquan McDonald. Police have said McDonald refused to drop a knife when officers confronted him while responding to a call about a man with a knife walking down a street on the city's southwest side in October 2014.

Attorney General Lisa Madigan's office said the police department had "unsubstantiated" claims that releasing the footage to a journalist could hinder an ongoing investigation or deprive anyone of a fair trial. The letter also said police had no legal right to withhold the video because another agency, the Independent Police Review Authority, was conducting the investigation.

An attorney for McDonald's family, Jeffrey Neslund, who has seen the video, said the footage shows McDonald was armed with a small knife but walking away from police when an officer opened fire. He noted that McDonald's mother doesn't want the video released, because she fears it could spark violence in her Chicago neighborhood similar to the riots that erupted in Baltimore and Ferguson, Missouri, after police-involved deaths of black residents.

The Chicago City Council took the unusual step in April of approving a $5 million settlement with McDonald's family, even though the family hadn't sued, after being advised to do so by a city attorney who had seen the video.

An autopsy report showed that McDonald was shot 16 times, including at least twice in his back. The autopsy report also said McDonald had PCP, a hallucinogenic drug, in his system.

Police have said that the officer who shot McDonald had been stripped of his police powers and assigned to desk duty. Police have released few details about the shooting, citing the ongoing investigation, but the city's attorney has said that McDonald was walking away from police when he was shot.

Police spokesman Anthony Guglielmi declined comment on Wednesday when asked about Thursday's trial. He didn't immediately return phone messages later in the day seeking comment on the attorney general's letter.

A Cook County judge is expected to issue a ruling Thursday on a public records request filed by a freelance journalist seeking the video.