(CNN) -- 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.
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."
"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:
Was Pistorius wearing his prosthetics?
Pistorius, who was born with a congenital abnormality and had both legs amputated before his first birthday, says he wasn't wearing his prosthetics at the time of the shooting.
Prosecutors say the sprinter's act was a premeditated murder -- that he took the time to put on his legs before walking to the bathroom and shot through the toilet door, intentionally killing his girlfriend.
What will ballistic tests on the door show? It will be crucial to hear the evidence from police experts on where the bullets hit the door and at what angle. Will the tests on the toilet door show that the trajectory of the bullets came downwards from a higher angle than would be possible if Pistorius was firing from his stumps? If true, it could bolster the state's case that Steenkamp's killing was premeditated. Or will Pistorius' version of the story -- that he was on his stumps -- be borne out by forensics?
Were Pistorius and Steenkamp arguing?
Pistorius says he and Steenkamp enjoyed a quiet dinner at home on the night of her death. "She was doing her yoga exercises and I was in bed watching television," he said in his court affidavit last year. "We were deeply in love and I could not be happier."
But authorities say a witness living 300 meters from Pistorius' home heard the couple having a heated argument in the early hours of Valentine's Day -- an assertion the track star's team disputed at his bail hearing.
Will phone data be the smoking gun?
The cell phones found at the scene could make or break Pistorius' case. Modern GPS technology can trace a phone's location within a meter as it is carried from one place to another. Steenkamp had taken her phone with her when she went to the bathroom. Did she try to call anyone in the moments before her death? Was Pistorius really in bed at 10 p.m., as he has claimed? The phone data may be able to tell the story of the couple's movements that night.
At last year's bail hearing in Pretoria, the magistrate also castigated police for failing to trace phone calls from Pistorius' home. But phones were far from the only problem the state had in laying out its case.
South African police in the spotlight
Hilton Botha, the lead investigator at the time, admitted that investigators contaminated the crime scene by walking through the house without wearing protective foot covers because they had run out of them.
Botha also testified that investigators had found two boxes of testosterone and needles at Pistorius' home -- to audible gasps from the packed courtroom -- before backtracking and admitting he may have misread the label. The defense said it was an herbal remedy.
The longer the hearing went on, the more holes appeared to show in the prosecution's case. At one point Pistorius' attorney forced a beleaguered Botha to admit that the state had thus far found no evidence to contradict the runner's version of events. Police in Pretoria soon pulled Botha off the case.
Pistorius has never denied killing Steenkamp. "I cannot bear to think of the suffering I have caused her and her family, knowing how much she was loved. I also know that the events of that tragic night were as I have described them," he claims in his affidavit.
If he isn't convicted of premeditated murder, the sprinter would still face a lesser charge of "culpable homicide," a crime based on negligence, and could be looking at up to 15 years in jail, experts say.
Pistorius is not claiming self-defense; he is claiming to have been mistaken about his need for self-defense. He is denying that he intentionally unlawfully killed Steenkamp.
If the court were to rule that his mistake was unreasonable -- based on what an objective, ordinary South African would do in the circumstances of the accused -- he would be found guilty of culpable homicide.
The first round of the trial, which begins on Monday, is expected to last three weeks. South Africa abolished jury trials in 1969, and Pistorius' case will be heard by Thokozile Matilda Masipa, only the second black woman appointed to the bench since the end of apartheid.
The case has shocked the world and put the spotlight on South Africa's rampant gun violence and high crime rates. Roughly 45 people are murdered every day, according to police statistics, and the number of home burglaries is up 70 percent in the last decade. In 2012, more than half of South Africans told the country's police force that they're afraid of having their homes broken into. In his affidavit, Pistorius claimed he had been the victim of violence and burglaries before, including death threats.
The trial has also divided opinion in South Africa. Many of his fans find it difficult to believe a national hero could be capable of the crime he's been charged with. Others find his version of events that night implausible, and point to troubling details that the case has dredged up about Pistorius' past.
In its indictment, police described Pistorius recklessly shooting a 9mm handgun out of the sunroof of a car on a public road in 2010. Police have also charged him with unlawfully firing a Glock 27 pistol at a restaurant in January of 2012. A friend of the sprinter told CNN that Pistorius was showing the gun to a friend when it went off at the outdoor café accidently.
Since Steenkamp's death, Pistorius has shied away from the spotlight. He lives at his uncle Arnold Pistorius' house in Pretoria, and although video surfaced last year of him training, he has not returned to the track in an official capacity. But support from his family has been unwavering.
"We have no doubt there is no substance to the allegation and that the state's own case, including its own forensic evidence, strongly refutes any possibility of a premeditated murder or indeed any murder at all," said Arnold Pistorius.
Hundreds of miles from Pretoria, Reeva Steenkamp's parents now run a pub called the Barking Spider in Port Elizabeth. Some of her family members, including her mother, have said that they will attend the trial. It will be the first time they'll face their daughter's killer in court. They have not been at previous court appearances because they said they wanted privacy. Now they are seeking "closure, and to know that our daughter did not suffer on that tragic Valentine's Day," the parents said in a prepared statement in mid-February.
In the meantime, Steenkamp's family has tried to focus on the memory of Reeva -- not on the media and legal circus about to unfold in Pretoria.
"It's not about the court case," her uncle, Mike Steenkamp, told CNN. "It's about Reeva, and Reeva can never be part of it."
At the same time, Mike Steenkamp said seeing Pistorius may bring at least some closure to what has been a harrowing year for the family.
"I would like to be face to face with him and forgive him, forgive him for what he's done. That way I can then probably find more peace."
CNN's Robyn Curnow, Ashley Fantz, Susannah Cullinane and CNN Wires contributed to this report.