CHICAGO (CBS) — The firs hours of testimony began Monday in the trial of Chicago Police Officer Jason Van Dyke. Each side took about 20 minutes for their opening statements, as Van Dyke watched, rarely changing his expression. Eight witnesses took the stand.

Jurors watched a new video depicting a slow-paced pursuit of Laquan McDonald. Another police officer can be seen trailing McDonald for blocks without ever firing his weapon. Prosecutors say that is because it was not necessary.

The never-before-seen video showed Laquan McDonald trailed by Officer Joseph McElligott, who appeared to be walking calmly alongside a squad car for several blocks.

“Why didn’t you fire your weapon?” asked the judge.

“We were trying to buy time to have a taser,” Officer Joseph McElligott replied.

In contract, said prosecutors, Jason Van Dyke rolled onto the scene, jumped from his squad car, and six seconds later pulled the trigger for the first time and began shooting Laquan McDonald.

“For the next 12.5 seconds, the defendant continues to pull the trigger of his gun over and over until he empties the entire clip. You’ll see it was not necessary to shoot Laquan McDonald a single time,” said Special Prosecutor Joe McMahon.

Jason Van Dyke’s Attorney, Dan Herbert, said, “They are writing the final chapter without showing you the rest of the book. We are going to show you the context.”

Herbert said McDonald was on a wild rampage through the city that night, high on PCP, and breaking into trucks. Jurors were shown photos of a tire he allegedly slashed on a squad car and the windshield attacked with a knife.

“Jason Van Dyke had reason to believe Laquan McDonald was going to hurt somebody,” Herbert stated. “This is a tragedy. Not a murder.”

Prosecutors contend Van Dyke could not, in those six seconds before he fired, have had the information he need to fire his weapon 16 times; but Van Dyke’s attorney said the number of shots are not important if the decision to shoot was lawful.

The prosecution told the jurors Van Dyke shot the teenager despite being surrounded by nine armed officers and five squad cars. His defense attorney said evidence will later show that significant physiological changes occur when an officer is engaged in a deadly force situation.

Court resumes in the Leighton Criminal Court Building Tuesday at 10 a.m.

CBS 2’s Roseanne Tellez and Dana Kozlov contributed to this story.