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William Porter faces charges of second-degree assault, involuntary manslaughter, reckless endangerment and misconduct in office. He has pleaded not guilty to all of the charges, as have the other five officers charged in the death of Gray, who died while in police custody in April, prompting protests in Baltimore.
Porter was calm and composed when he took the stand, talking about his background on the force and the series of events that took place on the day Gray was taken into custody.
Porter said that he knew Gray "from the neighborhood" and had run-ins with him prior to the day in question.
"Sometimes he would actively resist [arrest], he would usually act out and yell and fake some type of injury," Porter said.
Porter said that the reason he didn't call for medical assistance was because "after talking to Mr. Gray, he was unable to give me any reason to." Porter added that "medics usually take a while" to arrive.
"He never made a complaint of an injury," Porter said.
"He did not appear to be in any pain he looked tired, lethargic," he said.
When he saw that Gray was unconscious in the back of the police van when it made its final stop, Porter testified that he saw that Gray was "unconscious."
"It was a very traumatic thing for me also," Porter said.
The prosecution rested its case against Porter on Tuesday, arguing that Porter did not follow department rules requiring him to secure Gray with a seat belt when he was detained in the back of the police van in April.
As a result, Gray, whose hands and legs were shackled, suffered a severe spinal injury as he was likely tossed about while the van was en route to the police station, the prosecution argued.
Porter is expected to be one of the main witnesses to testify for the defense, though the exact order of the witnesses to be called has not been announced.
The other detainee who was in the van with Gray is expected to take the stand Friday.