Several officers, including the one now charged with murder in Laquan McDonald's death, reported that McDonald approached officers while armed with a knife.
However, squad car video released last week shows McDonald veering away from officers down a four-lane street in October 2014 before he was shot 16 times.
It shows officer Jason Van Dyke opening fire from close range and continuing to fire after McDonald crumples to the ground.
"In defense of his life, Van Dyke backpedaled and fired his handgun at McDonald, to stop the attack," one document reads. "McDonald fell to the ground but continued to move and continued to grasp the knife, refusing to let go of it."
The details emerged in hundreds of pages of handwritten and typed reports that prompted supervisors to rule McDonald's death a justifiable homicide hours after he was shot.
The release of the footage triggered protests and calls for public officials, including Mayor Rahm Emanuel, to resign.
Illinois Attorney General Lisa Madigan has called for an overall federal probe of police department practices, which top Democratic presidential candidates to local Illinois politicians have echoed. Emanuel has since fired the police chief, expanded a body camera program and formed a task force.
"If the criminal investigation concludes that any officer participated in any wrongdoing, we will take swift action," he said in an emailed statement.
Messages left for the authority, Emanuel's spokeswoman, Cook County State's Attorney Anita Alvarez' spokeswoman and a police union weren't immediately returned.
