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The chief prosecutor in Oscar Pistorius' murder trial urged the Olympic athlete to "take responsibility" for killing his girlfriend, imploring him on Wednesday to look at a photograph of Reeva Steenkamp's bloodied head displayed in court.

Prosecutor Gerrie Nel maintains the athlete killed Steenkamp deliberately after the two had an argument, saying her head "exploded" when it was struck by one of four bullets that the double-amputee fired through a closed door in his home last year. The photograph — shown during the first day of cross-examination of the star athlete — showed a side view of Steenkamp's head, with a mass of blood and human tissue on the back and upper parts. Her eyes were closed.

"It's time that you look at it," Nel said Wednesday.

Pistorius, 27, whose defense team maintains that he mistook Steenkamp for a burglar, said he didn't have to look because he was at the scene when she died.

"I remember," Pistorius said, becoming distraught and turning away from where the photo was shown on a TV screen next to him. "I will not look at a picture where I'm tormented by what I saw and felt that night. As I picked Reeva up, my fingers touched her head. I remember. I don't have to look at a picture, I was there."

The prosecutor showed a video, earlier broadcast on Sky News, of the celebrated Olympic athlete firing a gun at a watermelon and then saying it was "softer than brains" and calling the powerful .50-caliber handgun a "zombie stopper." Defense lawyer Barry Roux had earlier objected to the gun video being shown, saying it was inadmissible character evidence and amounted to a legal "ambush" of the defense. Judge Thokozile Masipa allowed the video to be shown.

Referring to the watermelon video, Nel said to Pistorius: "You know the same happened to Reeva's head? It exploded."

Pistorius, his voice rising and starting to sob, said he was at the scene when Steenkamp died and knew of her terrible head injury.

Nel began a rigorous cross-examination by asking Pistorius to explicitly acknowledge that he had killed Steenkamp.

"I made a mistake," Pistorius said.

"What was your mistake?" Nel replied.

Pistorius then said he "took Reeva's life," before Nel asked the athlete to say he shot and killed her.

"You killed her," Nel said. "You shot and killed her," and he asked Pistorius to say it. Pistorius would not, saying merely: "I did."

Earlier Wednesday, while under direct examination from his attorney, Pistorius testified that Steenkamp died in his arms after he shot her in the bathroom of his upscale home in a Pretoria gated community.

The double-amputee runner returned to the witness stand after weeping and wailing Tuesday while describing the moments he said he realized he had fatally shot his girlfriend by mistake. He said he thought an intruder was in the house.

Pistorius mostly kept his composure Wednesday, though he often paused and his voice quavered in apparent distress while describing what he said were his desperate attempts to help Steenkamp, a 29-year-old model, after shooting her through the closed bathroom door.

"Reeva had died while I was holding her," Pistorius said after describing how he put his fingers in her mouth to try to help her breathe and put his hand on her hip to try to stop bleeding from one of several gunshot wounds.

Pistorius said he felt "helpless," and that he wanted to take Steenkamp to a hospital, but a neighbor advised him to wait for paramedics to arrive. Police later appeared on the scene.

"I asked a policeman if I could wash my hands because the smell of the blood was making me throw up," Pistorius said. "Every time I saw Reeva, I got sick. So I stayed more inside the kitchen. I sat in the pantry against the washing machine."

The prosecutor asked Pistorius if people looked up to him as a sporting hero, if he wouldn't hide anything and if he lived by Christian principles.

"I'm here to tell the truth, I'm here to tell the truth as much as I can remember," Pistorius said. He also said: "I'm human. I have sins."

Pistorius is charged with premeditated murder and could face 25 years to life in prison if convicted of that charge. He broke down Tuesday as he recalled bashing open the bathroom door through which he shot Steenkamp in the early hours of Feb. 14, 2013. He sobbed and howled in court, causing the trial to be adjourned.

The Associated Press contributed to this report.