Stephen Roach paced back and forth in the dark alley. The Cincinnati police officer had just shot Timothy Thomas, who lay dying with a bullet through his aorta. He'd just learned the suspect held no weapon.

        His face went pale, his eyes stayed wide. Repeatedly, fellow Officer Chris Schroder asked him: “Are you OK?”

        Officer Roach didn't answer.

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        The officer's actions early that April 7 morning in Over-the-Rhine were the focus of testimony Monday, the first day of his trial on misdemeanor charges of negligent homicide and obstructing official business.

        It was the first time key details became public in the shooting that fueled the protests and riots April 9-12, and polarized the city.

        Court TV is broadcasting the trial live. Other than that, the proceedings were relatively low-key.

        A few of Officer Roach's fellow officers trickled in to pat him on the shoulder. One offered a mint, another told Officer Roach to keep his head up.

        Protesters outside the courthouse numbered three. Most reporters were kept in a side room to watch the testimony on small televisions.

        A key issue is whether the officer's 9 mm semiautomatic pistol “just went off,” as Officer Schroder said he heard Officer Roach explain at the scene.

        Or did the four-year veteran cop believe his life was in danger from a man whom witnesses said repeatedly had to grab the waistof his baggy pants as he fled police to keep them from falling down?

        Prosecutors say their evidence will show that only Officer Roach felt so threatened by Mr. Thomas that he drew his weapon, that others realized the 19-year-old man was merely pulling up baggy pants. Instead of telling the truth about what happened, prosecutors said, the officer tried to blame the dead, unarmed man.

        They also say he failed to follow his firearms training rules, including one that requires officers to keep their finger outside the trigger guard — to prevent the kind of startled reaction that might have happened in this incident.

        The defense doesn't dispute many of the facts, just their interpretation.

        Officer Roach's attorney, Merlyn Shiverdecker, intends to present Judge Ralph Winkler a picture of a kid who had wanted to be a cop since he was 4.

        Officer Roach displays maturity beyond his experience level, his lawyer said, and “is of such a type that would not ... fire his weapon unless he felt he were justified.”

        The obstruction charge stems from detectives' claim that Officer Roach changed his explanation of what happened that night.

        In interviews, he initially said he had not yet taken his gun out of its holster when he confronted Mr. Thomas behind an abandoned Republic Street building and ordered him to show his hands. He said he warned Mr. Thomas again, then drew his gun.

        He told detectives Mr. Thomas then raised his clenched fist in “a threatening manner.” Officer Roach told them he saw a dark object in Mr. Thomas' hand that he perceived as a gun. He fired.

        Detectives found discrepancies after reviewing a videotape of the incident recorded by a cruiser camera and from talking with other officers at the scene.

        Three days later, prosecutors said, Officer Roach told investigators he took his gun out as he walked down the alley. Detectives said the officer then explained: “He startled me and it just went off.”

        If convicted, Officer Roach, 27, could be sentenced to nine months in jail.

        Half the courtroom Monday was filled with his friends, family and in-laws.

        Just before opening statements, Officer Roach's wife, Erin, got up from her front-row seat, hugged him from behind as he sat and whispered in his ear.

        Fraternal Order of Police President Keith Fangman, sitting behind the family, said it was important to show support because Officer Roach is the first of three Cincinnati officers to go on trial this fall in suspects' deaths.

        Across the courtroom aisle,Angela Leisure, Mr. Thomas' mother. broke down into tears as special prosecutor Stephen McIntosh played a tape of officers' conversations that morning, particularly when the dispatcher repeated: “The suspect is down.”

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