Slager, who is white, is accused of killing Scott, an unarmed black man, at a traffic stop on April 4, 2015, in North Charleston, South Carolina, when Slager was a North Charleston Police Department officer. Witness video that surfaced shortly after appears to show the moment Slager fatally shot Scott as he ran away. The video garnered national attention, propelling Slager into the spotlight.
Slager, 34, has pleaded not guilty to murder. Slager told investigators he shot Scott because, while they were on the ground, he said Scott grabbed his Taser and pointed it at him.
Feidin Santana, 25, who recorded the video, was called to testify by the prosecution today, recounting the moments before, during and after he witnessed the deadly shooting.
Santana said he was walking to work when he saw "a man in a green shirt, identified as a black man, running towards me."
He said a "few seconds later I saw a second person, an officer, chasing the guy."
"I froze. Because I didn't know what was going on," he said. "I decided to stop and see what was happening."
"I decide to walk a little bit more and they enter to an empty lot," he said. "And that's when I ... start running to get there."
Santana said he went to the alley and could see the two men. Scott was on the ground, he noted.
Santana said an "electric sound" caught his attention as he approached, "and a punch from the officer to the person that was on the ground."
He said Scott was making "an uncomfortable sound."
He said he decided to approach the scene and start recording.
"It was a lot of movement from both men," he said. "I continued hearing the electric sound as I was approaching to the fence. They were moving a lot. The black man ... he just tried to get away from the Taser that I was hearing. But I didn't know [at the time] that it was a Taser -- I just knew that it was something electric sound."
Santana said the officer's left hand was trying to control Scott and the officer was on top of him. He said he never saw Scott on top.
Then both men stood up, Santana said, and he saw "the black man trying to get away and the officer trying to control the person."
"Both men get up very quick," he said. He said the officer was holding onto Scott but Scott was able to get away.
"After he got away ... it's been something that I didn't expect," Santana said. "[The officer] shoot the man running from him. And he shoot until he gets on the ground."
Santana said he continued recording and when the shooting was over, he walked toward Scott's body.
When asked by the prosecution if he noticed an object on the ground dropping between the two men, Santana said, "I saw something fell. I didn't know anything what it was."
When asked by the prosecution what the officer did, Santana said, "He went there and arrested him. I hear him saying 'put your hands on the back.'"
But Scott wasn't moving, Santana said. "He handcuffed him," Santana said.
Then the officer bent down and picked up the object, Santana said.
The officer who shot Scott, who Santana identified in court as Slager, then walked back toward Scott, Santana said, and more officers showed up at the scene.
Santana said another officer started checking on Scott.
Santana said he stopped recording and talked to a black officer at the scene to find out if Scott was OK.
"I went over there and asked the black officer if he was OK. And he gave me, I would say, a positive response," Santana said. He said the officer told him Scott was OK and told him to leave.
Santana continued recording for about a minute, he said, as other officers arrived. He said he told police that he witnessed what happened.
Santana's footage was played in court, showing Scott running away, Slager firing multiple shots and Scott falling to the ground. As the video was played, the families of Scott and Slager both appeared calm.
Santana told the court he did not alter his videos. "I'm a barber, not a technician," he said.
The prosecution asked if Santana saw Scott going toward Slager with a Taser and Santana said no.
Slager's attorneys have said the video doesn't show the whole struggle between Slager and Scott and does not give the perspective of events from Slager's point of view.
During cross-examination, the defense asked if Santana heard Slager yell "Taser" and Santana said no.
"As soon as I got there, the electric sound is what made me locate them," he said.
The defense asked if he saw a fight between Slager and Scott. "There was no fight," Santana testified.
He said he saw the officer on top controlling Scott.
"Scott was trying to get away from the Taser and the officer was in position of control," he explained.
The defense asked Santana who -- Slager or Scott -- was holding the Taser. Santana said he didn't see the Taser in Scott's hands. He said he could clearly see one of Slager's hands even though there was a lot of movement.
The defense asked Santana if he heard Slager announce that shots were fired and if Slager had called out that the suspect was down. Santana responded that he heard talking but he did not know what was said.
The defense asked Santana if any more shots were fired after Scott was shot and fell to the ground. Santana replied, "When he got on his knees and he fell down completely I didn't hear any shots."
Santana did say that after the shooting Slager had told Scott to put his hands on his back.
When the defense asked Santana if he tried to alert emergency services to help Scott, he said no because he thought police would be doing that as part of their job.
The Associated Press contributed to this report.