PRETORIA, South Africa -- Testimony in the murder trial of Oscar Pistorius focused Tuesday on a pillar of the prosecution's case -- the screams that neighbours heard on the night that the athlete killed girlfriend Reeva Steenkamp.

Chief prosecutor Gerrie Nel questioned an acoustics expert who had been called by the defence team as part of its effort to suggest some neighbours who said they heard the screams of a woman were wrong, and that they actually heard the high-pitched screams of the double-amputee runner.

Several neighbours called by the prosecution have testified they heard a woman's terrified screams on the night Pistorius shot Steenkamp, which could bolster the prosecution's claim that the couple were arguing before Pistorius opened fire. The defence has suggested the screams came from Pistorius after he realized he had mistakenly shot Steenkamp.

Pistorius fatally shot Steenkamp through a closed toilet door in his home in the early hours of Feb. 14, 2013. He has testified that he fired in the mistaken belief there was a dangerous intruder in his home. The prosecution has alleged that Pistorius killed Steenkamp after a Valentine's Day argument.

The acoustics expert, Ivan Lin, has testified that he conducted tests that showed ambient noise and other factors can make it difficult to hear accurately from a distance.

Nel said the screams of a woman have a "tonal character" and referred to the testimony of the neighbours who were convinced they had heard a screaming woman.

Lin responded that he could not say whether the neighbours were "correct or incorrect."

Pistorius faces 25 years to life in prison if found guilty of premeditated murder, and could also face years in prison if convicted of murder without premeditation or negligent killing. He is free on bail.

On Monday, the court received reports from mental health experts who concluded Pistorius was not suffering from a mental illness when he killed Steenkamp and was able to understand the wrongfulness of what he had done. The reports were compiled during a month of tests at a state psychiatric hospital.