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Officer Jeronimo Yanez said in an interview following the fatal traffic stop that Castile's apparent willingness to use the drug in front of his young daughter and girlfriend led Yanez to believe that the 32-year-old posed a serious threat.
It shows the two speaking calmly, with Castile heard saying: "Sir, I have to tell you that I do have a firearm on me".
Yanez fired seven bullets at Castile, hitting him five times, after ordering the school cafeteria supervisor to pull his vehicle over because of a broken brake light.
He told Mr Castile he pulled him over because a brake light was out before he asked for his driver's licence and insurance.
The 29-year-old police officer was acquitted last Friday of second-degree manslaughter and of two felony counts of intentional discharge of a unsafe weapon. The jury in the case reportedly saw the dashcam footage before reaching its verdict. At this, Yanez is seen laying his right hand on the gun in his holster.
The video was played during Yanez's trial.
"I have to tell you I do have a firearm on me", Castile said.
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It has been a spectacular day. "I made the podium for the first time so I will drink out of anybody's shoes". I was flat out and when you are chasing other cars you lose quite a lot of grip, then you are sliding.
But the professor says Yanez "made a awful mistake, and he shouldn't be a cop".
Castile responds, "I wasn't reaching".
After the shooting, the video shows Yanez standing at the auto window, breathing heavily and cursing repeatedly, with his gun drawn for some time.
Yanez fired seven shots total, hitting Castile five times.
When additional officers and an ambulance arrive on the scene, they remove Castile's body from the vehicle and attempt to tend to his wounds.
The jury's acquittal of Yanez - who told jurors he acted in self-defense when he opened fire on Castile - prompted thousands to protest last week in St. Paul, Minnesota. Yanez can be heard talking to other officers, who are asking him questions.
In the police dashcam footage, he can be heard telling Yanez about the "firearm" in a calm and orderly manner right before the cop starts blasting. Yanez, who testified he thought he was going to die, was acquitted of manslaughter.