A witness in the shooting of an unarmed black teenager by a white police officer said Wednesday he saw the officer standing on the sidewalk, panicking, saying, "I don't know why I shot him. I don't know why I fired." The trial of former East Pittsburgh police officer Michael Rosfeld continued into a second day Wednesday in a Pittsburgh courtroom, with three witnesses were called during morning testimony.

John Leach, a neighbor who lives a few houses away from the scene of the June shooting, said he was on the front porch when Rosfeld fired three bullets into 17-year-old Anton Rose II. Rosfeld had just pulled over an unlicensed taxicab suspected to have been used in a drive-by shooting minutes earlier. Rose had been riding in the front seat of the taxi when Zaijuan Hester, in the back seat, rolled down a window and shot at two men on the street.

East Pittsburgh Police Officer Michael Rosfeld (L) is charged with the shooting death of 17-year-old Antwon Rose Jr. (R). Allegheny County Jail (L); STK, Embrace Life/Woodland Hills School District via CBS Pittsburgh (R)

Hester, 18, of Swissvale, pleaded guilty Friday to aggravated assault and firearms violations for the earlier shooting, which wounded a man in the abdomen. Hester told a judge that he, not Rose, did the shooting.

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Rosfeld, 30, faces a charge of criminal homicide for shooting Rose as the teen fled the traffic stop.

Leach, the second witness to testify Wednesday, said he was standing on his front porch smoking when the shooting happened, reports CBS Pittsburgh. He said he saw Rosfeld fire three shots just as Rose turned and fled. Leach said he "couldn't believe his eyes." After the shooting, he was standing by Rose's body, watching Rosfeld on the sidewalk nearby saying repeatedly, "I don't know why I shot him. I don't know why I fired."

There were audible gasps in the courtroom during the testimony, the station reports.

A memorial display for slain teen Antwon Rose featuring flowers sand his portrait was outside a Pennsylvania court Tuesday as the trial launched for the former officer charged in his death CBS Pittsburgh

Leach said later, he saw other officers consoling Rosfeld as he was crying, bent over and hyperventilating. Rosfeld, he said, looked as if he was about to pass out.

Leach said he saw Rosfeld pointing a gun at Rose while at least one of Rose's hands was in the air. Then Rose turned and ran, he said.

Patrick Shattuck said Wednesday he was in a senior center across the street for a council meeting. Five to seven minutes after the shooting, Shattuck said Rosfeld, with swollen, red eyes, entered the building and said, "Why did he do that? Why did he do that? Why did he take that out of his pocket?"

East Pittsburgh Mayor Louis J. Payne, who was also there, said he, too, heard Rosfeld say, "Why did he do that?" but said he didn't hear the comment about the pocket.

Defense attorney Patrick Thomassey said Rosfeld did not intend to shoot anyone that day and did nothing wrong in his fatal encounter with Rose.

Former East Pittsburgh officer Michael Rosfeld enters a Pennsylvania courtroom for a pre-trial hearing in the death of unarmed teen Antwon Rose CBS Pittsburgh

"You think Michael Rosfeld got up on the 19th of June and thought he was going to shoot someone? Of course not," he said.

In its opening statement, the defense asserted that Rose was complicit in the drive-by shooting by identifying the target to the gunman, Hester. Rose had an empty ammunition clip in his pants when he was killed, and two handguns were recovered from inside the vehicle.

But prosecutors urged jurors to focus on what Rosfeld knew about any of that when he shot Rose, and said Rosfeld gave inconsistent statements about the shooting, including whether he thought Rose had a gun.

Michelle Kenney, mother of Antwon Rose, enters a Pennsylvania court Tuesday as the trial opens for the former cop who fatally shot her son last year CBS Pittsburgh

The video of the shooting, recorded by neighbor Lashaun Livingston, was posted online, triggering protests in the Pittsburgh area last year, including a late-night march that shut down a major highway.

A jury of six men and six women, including three African-Americans, was selected across the state in Harrisburg last week and will be sequestered in a Pittsburgh hotel for the duration of the trial, which is expected to take a week or more.

Rose's mother sent a letter to prosecutors Wednesday urging them to counter the defense's portrayal of her son as "just another thug." In the letter, she asks prosecutors to paint a picture of her son as he truly was.

"He was a rose that grew from concrete. Despite darkness all around him, he was kind, loving and funny," she wrote in the letter dated Tuesday.