CHICAGO — A doctor testifying in the trial of a white Chicago police officer charged with murder in the death of a black teenager said Wednesday that it's impossible to determine the exact order of the wounds Laquan McDonald suffered from 16 shots that were fired.

Dr. Ponni Arunkumar, Cook County's chief medical examiner, testified about each gunshot wound as jurors viewed autopsy photos of them. She said that when the entrance and exit wounds are added, there were "24 holes on Laquan." That's because eight of the shots left both exit and entrance wounds.

Officer Jason Van Dyke is charged with first-degree murder in McDonald's death. Video from the October 2014 shooting shows Van Dyke opening fire as McDonald walks away from police with a small knife in one hand.

Arunkumar said it's impossible to determine the order of the gunshots in such cases because the shooter and the person being shot are in "constant motion."

She was among several witnesses who testified Wednesday about the science behind McDonald's death. A paramedic who brought McDonald to a hospital and an emergency room nurse who wrote down all of the things doctors said as they examined McDonald both described the medical care he received. An Illinois State Police forensics expert testified about bullets and shell casings found at the scene, saying they were all from the same gun.

Prosecutors have stressed no other officers who encountered McDonald opened fire. Van Dyke's attorneys say he was afraid for his life and acted as he was trained.