(CNN) Laquan McDonald's face was expressionless -- "his eyes were just bugging out of his head" -- as the teenager kept "advancing" on him, holding a knife, Chicago police Officer Jason Van Dyke testified Tuesday in his murder trial.
The 17-year-old had "huge white eyes, just staring right through me," said Van Dyke, who is charged with killing McDonald in October 2014. Standing about 10 to 15 feet away, McDonald "turned his torso towards me," the officer testified.
"He waved the knife from his lower right side, upwards, across his body, towards my left shoulder," the officer said, appearing to get choked up as he demonstrated the action to jurors.
Van Dyke, who took the stand for the first time, told jurors it was then that he shot McDonald. But McDonald still refused his repeated commands to drop the knife, even as he lay wounded, Van Dyke said. McDonald appeared to be trying to get up after he stopped shooting, so he reloaded -- as he was trained to do -- and fired at the knife, Van Dyke said. But a prosecutor pointed on inconsistencies in Van Dyke's depiction of the fatal encounter.
"I could see him starting to push up with his left hand off the ground. I see his left shoulder start to come up. I still see him holding that knife with his right hand, not letting go of it," Van Dyke said. "His eyes are still bugged out. His face has got no expression on it."
He told jurors: "I just kept on looking at the knife, and I shot at it. I just wanted him to get rid of that knife."
Van Dyke faces two counts of first-degree murder, 16 counts of aggravated battery and one count of official misconduct in McDonald's death. Van Dyke is white and McDonald was black. Prosecutors say Van Dyke fired unnecessarily within six seconds after arriving at the scene, striking McDonald 16 times.
The shooting was captured on a grainy police dashcam video. Van Dyke said he fired in self-defense after McDonald lunged at him with a knife. But the dashcam video -- which a judge ordered the city to release 13 months after the shooting -- showed McDonald walking away from police, rather than charging at them.
During cross examination, Jody Gleason, one of the prosecutors, tried to challenge Van Dyke's assertion that McDonald raised the knife.
"You've sat here for several days and watched several videos. ... Have you ever seen Laquan McDonald do that on one of those videos?" Gleason asked.
Van Dyke said the dashcam video of the shooting an an animated recreation of the shooting presented by the defense didn't show his perspective. But he acknowledged he didn't see McDonald raising the knife in the recreation.
Van Dyke said the dashcam video "may not show" McDonald trying to get up after he was shot. "I was coming from a completely different perspective," he said.
Van Dyke told Gleason he decided to stop shooting when he realized McDonald had hit the ground. He said he lowered his weapon, reassessed and continued firing "on my approach."
"So, as you're approaching him while he's on the ground, you're continuing to shoot him?" Gleason asked.
"You're putting words into my mouth," Van Dyke said.
He later repeated that he shot at McDonald's knife, trying to get rid of it. But Van Dyke also acknowledged police officers are not trained to shoot at someone's knife.
"So why would you continue to shoot at his knife? That's not what you're trained to do," Gleason said.
"My focus was just on that knife, and I just wanted him to get rid of that knife. That is all I could think," Van Dyke said.
Van Dyke said several times that he could not recall what he told a fellow detective shortly after the shooting about McDonald's actions.
"I was under a lot of stress immediately after the shooting," he said.
Gleason asked Van Dyke if he knew an officer with a Taser was on their way when he encountered McDonald.
Van Dyke said yes, but then he just heard a car had responded, coming from a distance. But he never heard over the radio that a Taser unit was on its way, he said.
Gleason asked when Van Dyke got out of his squad car, he could have called and asked where the officer with the Taser was.
Van Dyke said, "hypothetically anybody on the scene" could have made that call, but it was a chaotic moment. He couldn't answer yes or no, he told Gleason.
"You didn't even have to get out of the car at that point, did you?" Gleason said.
Van Dyke told her: "As an officer, you have a duty to not retreat."
He said he didn't have time to find a barrier between him and McDonald to protect himself in the seconds before he fired, Van Dyke said.
"In that six seconds, he got a lot closer to me than I ever could have gotten away from him around the squad car," he said.
"And you got a lot closer to him, too, didn't you?" Gleason said.
Van Dyke said: "I know that now, yeah, not intentionally."
The officer said he thought he "was backpedaling that night."
"You thought you were backpedaling as you were firing shot after shot after shot?" Gleason asked.
"What I know now and what I thought at that time are two different things," Van Dyke said.
The dashcam video of the shooting shows McDonald walking toward a fence on his right, as Van Dyke's squad car pulls up, facing McDonald, to the teenager's left.
The release of the footage sparked protests, a Justice Department civil rights investigation, criticism of the city's mayor, Rahm Emanuel, and eventually the ouster of the police superintendent.
During redirect, Van Dyke told defense attorney Randy Rueckert that McDonald could have "made a decisive turn and walked in the opposite direction."
"He could have thrown that knife away and ended it all right then and there," Van Dyke said.
Van Dyke wrapped up his testimony Tuesday. The defense is expected to rest its case on Wednesday.
Van Dyke is the first Chicago officer to be charged with first-degree murder since 1980. He could face up to life in prison if convicted.