In many recent, high-profile prosecutions of police shootings, officers testified on their own behalf, though experts said the number of such cases is too low to draw many conclusions. The outcomes, too, have been mixed. Last year, police officers who testified in shooting trials in Minnesota, Missouri and Oklahoma were acquitted. Another officer in Wisconsin was acquitted without testifying. And this summer, an officer in Texas was convicted of murder after taking the witness stand.

Often, police officers have used similar language on the witness stand to defend their actions. In the Minnesota case, Officer Jeronimo Yanez said he “had no choice” but to shoot at Philando Castile, a black motorist. In the Oklahoma case, in which an officer shot Terence Crutcher, an unarmed black man, Officer Betty Jo Shelby told jurors that she believed he had a gun. “I meet a gun with a gun,” she said. And in Ohio, where two juries failed to reach a verdict in the fatal shooting of Samuel DuBose, Officer Ray Tensing said last year that he opened fire after “my police instinct kicked in.”

In the Chicago courtroom on Tuesday, Officer Van Dyke described hearing a colleague radio for help after a report of someone breaking into vehicles, and racing to the scene that evening in 2014. He said he became worried after learning that Laquan had popped the tire of a police cruiser, presumably with a three-inch folding knife he was carrying. When Officer Van Dyke arrived at the scene, he said Laquan’s demeanor heightened his concern.

“His face had no expression. His eyes were just bugging out of his head. He had just these huge white eyes just staring right through me,” Officer Van Dyke said. “I was yelling at him, ‘Drop the knife!’”

The police dashboard camera video of the shooting did not capture audio of what took place, and the camera angle, from behind Laquan, did not show Laquan’s facial expressions or the front of his body. Officer Van Dyke began shooting within seconds of pulling up to the scene, the video showed. About 10 other officers on the scene, some of whom had been following Laquan for several blocks, did not fire their guns.

On cross-examination, prosecutors questioned Officer Van Dyke about why he did not wait for a colleague with a Taser to arrive, why he stepped toward Laquan as he was firing and why he said Laquan was approaching him when the video showed the teenager steering away, toward the other side of the street.

“You didn’t even have to get out of the car at that point, did you?” asked Jody Gleason, a prosecutor.