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Jason Van Dyke, the white Chicago police officer accused of fatally shooting a black teenager 16 times, took the stand on Tuesday in his first-degree murder trial to say that the young man, Laquan McDonald, was a threat from the moment he saw him.

“His eyes were bugged out – just expressionless, and he turned his torso toward me,” Van Dyke told jurors during his 90-minute testimony.

The trial over the 2014 killing has polarized Chicago and drawn national attention to the debate over how best to police minority communities.

Van Dyke is charged with first-degree murder, 16 counts of aggravated battery and a count of official misconduct in McDonald’s death. He was suspended without pay and has pleaded not guilty to murder.

Van Dyke joined the Chicago police force in June 2001 at the age of 23; he started his testimony by telling jurors about his early assignments as a beat cop.

At times emotional, Van Dyke, wearing a light blue tie matching the color of the department uniform, testified he'd never fired his weapon before the night he shot McDonald.

“I’m very proud of that,” he said.

Van Dyke said he and his partner, Officer Joseph Walsh, noticed McDonald walking down the street with a knife in his hand. Van Dyke said he thought McDonald dropped his knife and was “greatly relieved.”

But then things escalated quickly, he told jurors, asserting that McDonald was “advancing on me.”

“His face had no expression, his eyes were bugging out of his head. He had these huge white eyes just staring at me,” Van Dyke said, adding that McDonald was about 10 to 15 feet away by this point.

Van Dyke said McDonald waved the knife toward Van Dyke’s shoulder – that’s when the officer opened fire.

“Eventually he fell to the ground,” Van Dyke said. “Once I recognized he fell to the ground, I stopped shooting.”

Pushing back, prosecutors challenged his account of McDonald raising a knife before being shot.

“You’ve sat here for several days. Where do you see it on the video?” Assistant Special Prosecutor Jody Gleason asked Van Dyke. When he replied that the video didn’t show his perspective, Gleason showed Van Dyke an animation created by his own defense team. Van Dyke said he didn’t see his perspective on that model either, however.

Van Dyke’s attorneys painted McDonald as a dangerous teen who refused to drop a three-inch retractable knife he'd been carrying the night he was killed.

Defense attorney Daniel Herbert told jurors that McDonald was an “out-of-control individual who didn’t care about anyone,” arguing that his client was a “scared police officer who was fearful of his life and the life of others.”

Last week, a police academy teacher testified that Van Dyke was trained to shoot rapidly and immediately reload his gun.

The defense also called a pharmacology expert, who claimed McDonald was “whacked on this PCP.” The expert, James O’Donnell, testified that McDonald was more vulnerable to hallucinations because he didn’t take his prescribed medication – an antipsychotic and a mood stabilizer. His comments gave weight to the defense’s claims that McDonald was a danger.

Van Dyke claimed he shot McDonald 16 times when the teen swung a knife at him. Grainy dashcam video — released 13 months later after a court order — showed McDonald holding a knife at the side of his body, about 15 feet away from Van Dyke, and walking away from him and other officers who'd responded to a report that the youth was trying to break into vehicles. McDonald fell to the pavement less than two seconds after he was shot. Van Dyke continued shooting for another 12 seconds, emptying his 16-shot semiautomatic gun.

Prosecutors, who rested their case on Sept. 20, called the killing “completely unnecessary” and argued that race had been a factor. They claimed that on the night McDonald was shot and killed, the only thing Van Dyke saw was a “black boy walking down the street” who had the “audacity to ignore the police.”

Eury Patrick, the prosecution’s expert on deadly use of force by the police, testified that Van Dyke kept shooting “long beyond the point of being reasonable.”

“They’re not trained to just empty their gun,” Patrick said. “It’s not a knee-jerk reaction. They’re trained to shoot until the risk is ended.”

Witness Jose Torres told jurors he heard more gunshots after McDonald fell than before.

“I’m not going to use the word, but I said, ‘Why the "f" are they still shooting him if he’s on the ground?'”

Fox News' Ruth Ravve contributed to this report.

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