Oscar Pistorius: South Africa gears up for its 'trial of the century

He was one of South Africa's favorite sons, an amputee track star who defied all the odds and sprinted into the hearts of millions during the 2012 Summer Olympics in London. She was a staggering beauty with the brains to match, a law graduate and model whose star was on the rise.

Oscar Pistorius, 27, and his girlfriend Reeva Steenkamp, 29, were a young, attractive and high-profile couple who were popular in South Africa's social circles. The "Blade Runner," who won six Paralympic gold medals and was the first double amputee runner to compete in the Olympics, was an international superstar. Cover girl Steenkamp, soon to star in a TV reality show, was on the cusp of becoming a celebrity in her own right.

But everything changed before dawn on Valentine's Day 2013, as Steenkamp lay lifeless in a pool of blood on the floor of her boyfriend's house in an upscale gated community in Pretoria. Moments before, Pistorius says, he had pointed his 9mm Parabellum pistol towards an upstairs toilet room and fired four bullets through the locked door.

He says he mistook Steenkamp for a burglar. The state says it was cold-blooded murder.

When Pistorius steps once again into the spotlight Monday, more than a year after that fateful morning, it will be as the defendant in South Africa's "trial of the century." Hundreds of journalists from around the world will be in Pretoria to witness the proceedings get under way. Millions more will follow parts of the trial on live television, thanks to a judge's decision last week allowing cameras in a South African courtroom for the first time -- although any testimony by Pistorius, or witnesses who do not consent, will not be televised.

INTERACTIVE: Explore each side's argument

Pistorius faces one charge of premeditated murder in the shooting death of Steenkamp, a firearms charge associated with her killing, and two separate gun indictments from previous incidents. In South Africa, murder carries a mandatory life sentence.

The sprinter's lawyers will argue he was a man deeply in love with his girlfriend who made a terrible mistake. Pistorius says he heard a noise from the bathroom in the middle of the night and, feeling vulnerable without his prosthetic legs on, charged towards the bathroom on his stumps and shot through the toilet door in order to protect himself and Steenkamp.

"I felt a sense of terror rushing over me," he said in his court affidavit. "There are no burglar bars across the bathroom window and I knew that contractors who worked at my house had left the ladders outside."

"It filled me with horror and fear of an intruder or intruders being inside the toilet. I thought he or they must have entered through the unprotected window. As I did not have my prosthetic legs on and felt extremely vulnerable, I knew I had to protect Reeva and myself."

"I grabbed my 9mm pistol from underneath my bed. On my way to the bathroom I screamed words to the effect for him/them to get out of my house and for Reeva to phone the police. It was pitch dark in the bedroom and I thought Reeva was in bed."

READ MORE: Oscar Pistorius' affidavit to court in full

"I noticed that the bathroom window was open. I realized that the intruder/s was/were in the toilet because the toilet door was closed and I did not see anyone in the bathroom. I heard movement inside the toilet. The toilet is inside the bathroom and has a separate door."

"I fired shots at the toilet door and shouted to Reeva to phone the police. She did not respond and I moved backwards out of the bathroom, keeping my eyes on the bathroom entrance. Everything was pitch dark in the bedroom and I was still too scared to switch on a light. Reeva was not responding. When I reached the bed, I realised that Reeva was not in bed."

Pistorius claims he went back to put his prosthetic legs on, and found Steenkamp inside the bathroom slumped over. He says he carried her downstairs, where she died in his arms.

But prosecutors are painting a different picture. They believe the pair had an argument and that Steenkamp locked herself in the toilet. At last year's bail hearing the state said Pistorius put on his prosthetic legs, collected his gun from under the bed, and walked down the hall leading from the bedroom to the bathroom before unloading a flurry of shots through the door.

Observers believe the case will hinge on several pieces of evidence: