After a day of intense courtroom confrontation and emotion, Oscar Pistorius returned to the stand for a fourth straight day on Thursday to face new questions from a dogged prosecutor intent on highlighting even the most minute discrepancy in his version of what happened the night he killed his girlfriend, Reeva Steenkamp.

Mr. Pistorius, 27, has denied a charge of premeditated murder, which carries a minimum 25-year jail term, saying he shot Ms. Steenkamp, 29, by mistake, believing an intruder at his home was about to attack him when he fired four rounds from a handgun through a locked bathroom door while she was inside.

For the first time on Wednesday, Mr. Pistorius, a double amputee track star who competes on scythe-like prosthetic blades and who is the world’s best-known disabled athlete, faced the state prosecutor, Gerrie Nel, whose tactics have earned him the nickname “the pit bull.” The trial, in the South African capital, Pretoria, is being broadcast live, although Mr. Pistorius is kept off-camera under an earlier court ruling.

From the start, Mr. Nel set a pugnacious tone, challenging Mr. Pistorius to take responsibility for the killing, goading him in an attempt to undermine his composure and producing video images that showed him blasting a head-size watermelon with high-powered ammunition.

Then, in a move that brought gasps to the courtroom, Mr. Nel taunted Mr. Pistorius with a photograph of the bloodied, shot-open head of Ms. Steenkamp.

“That’s it — have a look, Mr. Pistorius!” the prosecutor said as Mr. Pistorius sat, stunned, in the witness box, seeming to crumple in on himself. “I know you don’t want to, because you don’t want to take responsibility, but it’s time that you look at it. Take responsibility for what you’ve done, Mr. Pistorius.”

But the runner refused to look. “I’m tormented by what I saw and felt that night,” he said. “As I picked Reeva up, my fingers touched her head. I remember. I don’t have to look at a picture. I was there.”

After the hours of cross-examination, Mr. Pistorius seemed drained but Mr. Nel left him in no doubt that there was more to come. “I’m not going to go away,” the prosecutor told the athlete.