The trial of Oscar Pistorius begins in Pretoria, South Africa with the Olympic and Paralympic track star pleading not guilty to murdering his girlfriend, Reeva Steenkamp. (Reuters)

“Bang . . . bang, bang, bang.”

The murder trial of Oscar Pistorius, 27, opened Monday with testimony from a neighbor who described the sound of what she said were four gunshots and recalled the “blood-curdling screams” of a woman who prosecutors say was the girlfriend killed by the onetime star athlete in his home.

“It’s the most helpless feeling I’ve ever had in my life,” university lecturer Michelle Burger said of listening to the screams. “I knew something terrible was happening in that house.”

Pistorius, a double-amputee runner whose stature peaked at the 2012 London Olympics and plummeted when he shot model and television personality Reeva Steenkamp in the pre-dawn hours of Valentine’s Day last year, stood in the dock in a dark-gray suit and black tie, writing on a pad of paper and sometimes passing notes to his attorneys. At one point, he smiled at a person sitting behind him. Steenkamp’s mother, June, sat near Pistorius, but there was no communication between them.

The proceedings were broadcast on television, though Burger was not shown at her own request, and millions of people around the world followed a trial where the heady mix of a celebrity defendant and shocking allegations has drawn comparisons to the O.J. Simpson case two decades ago.

Prosecutors allege that Pistorius, who has been free on bail, shot Steenkamp after an argument. He has said he killed her after mistaking her for a nighttime intruder in his home, shooting her through the closed door of the toilet cubicle in his bathroom. Steenkamp, 29, was hit three times — in the head, elbow and hip area; a fourth bullet did not hit her.

Early testimony focused on whether the screams that Burger said she heard were those of a terrified woman about to be shot to death, as prosecutors allege, or were instead Pistorius’s desperate shrieks for help after a fatal mistake, as the defense contends.

Burger, who lives about 195 yards from Pistorius’s house, gave her account of the sequence of events.

“I was woken up by a woman’s petrified screams. I heard her screaming first,” Burger said. “Then I heard her call for help. Then I heard a man call for help three times. I then made a call. . . . I gave the phone to my husband and he spoke to security. Afterward, I heard the woman’s petrified screams again.”

Burger said she then heard four gunshots, with a gap between the first shot and the rest, and more screaming.

“I heard her voice during the shots,” she said. “Shortly after the shots was the last time I heard that woman.”

Barry Roux, the chief defense lawyer, opened his cross-examination by asking Burger whether she thought Pistorius was a liar. She didn’t directly answer but questioned Pistorius’s version.

Graphic: What “Blade Runner” Olympian says happened the night he shot his girlfriend

“I can only tell the court what I heard that evening,” Burger said. “I cannot understand how I could clearly hear a woman scream but Mr. Pistorius could not hear it.”

Pistorius pleaded not guilty to charges of murder, as well as three other counts relating to firing guns in public in unrelated incidents and illegal possession of ammunition.

If convicted of the murder charge, Pistorius could be sent to prison for at least 25 years before the possibility of parole, the minimum someone must serve if given a life sentence in South Africa.