CHICAGO (Reuters) - A judge appointed a special prosecutor on Friday to investigate whether Chicago police officers lied to justify the shooting to death of a black teenager who was shot 16 times by a white officer, according to court documents.

Cook County Judge LeRoy Martin Jr. named former federal prosecutor Patricia Brown Holmes to probe whether charges should be filed against any Chicago police officers for their conduct following the 2014 shooting death of 17-year-old Laquan McDonald.

Dashboard camera videos of the shooting by officer Jason Van Dyke, 38, show McDonald walking away from officers. He was armed with a small knife. Van Dyke faces murder charges in the shooting, the first time in decades a Chicago police officer has been charged with committing such a crime while on duty.

Holmes will look into whether officers who witnessed the shooting described it in a way that would cover up misconduct, when they were interviewed after the incident. Police officers are justified in using lethal force if they were in reasonable fear that someone was threatening grievous bodily harm to themselves or another person.

Holmes, who is a former county judge and current partner at a Chicago based law firm, could not be reached by Reuters for comment.

"I plan to look at the facts and go from there," she told the Chicago Tribune. "I don't have any preconceived notions about how it's going to go or what I'm going to do."

The shooting came at a time of heightened national debate over police use of deadly force, especially against black men.

The incident sparked demonstrations and led to the firing of the police chief, an ongoing federal investigation of the police department and calls for Mayor Rahm Emanuel to resign.

Separately, another Cook County judge is in the process of appointing a special prosecutor in Van Dyke's first-degree murder trial.

(Reporting by Justin Madden; Editing by Fiona Ortiz and Bill Rigby)