"Tired" Oscar Pistorius faces more grueling cross-examination in murder trial

Last Updated Apr 11, 2014 7:57 AM EDT

PRETORIA, South Africa -- The prosecutor at Oscar Pistorius' murder trial on Friday challenged the athlete's statements that he was worried about crime before he fatally shot girlfriend Reeva Steenkamp through a closed toilet door in his home, and his claim that she never screamed as the shots were fired.

Chief Prosecutor Gerrie Nel told the world-famous athlete that he fatally shot his girlfriend through a toilet door as they were talking. Nel insisted that Pistorius intentionally killed Steenkamp and said Friday during cross-examination of the double-amputee Olympian that "she was standing behind the toilet door talking to you when you shot her."

Pistorius denied Nel's claim, saying "that's not true" in a dramatic exchange to end the first week of the runner's testimony at his trial.

Nel said the "only reasonable explanation" for the shooting last year was that Pistorius killed the model after a fight. Pistorius claims he shot Steenkamp by mistake thinking she was a nighttime intruder behind a toilet door in his bathroom in the early hours of Feb. 14, 2013.

He faces 25 years to life in prison if convicted of premeditated murder.

Nel examined the details of the alarm system at Pistorius' house, questioning why the athlete would believe an intruder had broken into his home when he had extensive security measures, including interior and exterior sensors. Pistorius said he activated the sensors on the home alarm system before going to sleep on the night he killed Steenkamp, but feared that building contractors doing work on his house may have moved some of the security beacons.

The prosecutor said Pistorius had not mentioned immediately after the shooting that he had fears that building contractors had removed some of the security beacons, specifically near the bathroom window where he allegedly thought an intruder may have gained access on the night he killed Steenkamp.

Querying why Pistorius had not mentioned those fears earlier, Nel said Pistorius was trying to build a story to explain his fears of an intruder and therefore a mistaken shooting.

"This is the biggest example of you tailoring your evidence," Nel said. Pistorius denied he was fabricating a story.

Pistorius also said he was struggling to give clear testimony because he was tired, prompting the judge to ask him if he was too tired to proceed with a tough cross-examination from the chief prosecutor.

Correspondent and CBS News contributor Debora Patta, who was inside the courtroom, said even the judge appeared frustrated Friday by

Pistorius was responding to a question from Nel, who pointed to a lack of clarity in the double-amputee runner's testimony about whether he turned off the alarm inside his home on the night of Feb. 14, 2013. Pistorius said he "must have" turned off the alarm, which Nel described as a vague response.

The prosecutor then asked Pistorius, who acknowledged making a mistake in his testimony, if he needed time before continuing with his testimony.

"I don't need time," the Olympic athlete said. "I am tired. It's not going to change."

Nel responded: "You're trying to cover up for lies and I'm not convinced."

Judge Thokozile Masipa interjected, asking Pistorius if he was too tired to proceed.

"You can be at a disadvantage when you're in that box," she said, adding that it wasn't fair to the court if he was not alert during the proceedings.

Pistorius replied that he was able to go on.