More than a year and a half after Oscar Pistorius shot and killed his girlfriend, Reeva Steenkamp, in his home in the middle of a dark February night, the world-famous sprinter's murder trial is racing for the finish line.

Judge Thokozile Masipa is set to deliver her verdict in the case Thursday, with possibilities ranging from a life sentence to letting Pistorius -- who's been nicknamed the "Blade Runner" for the prosthetic devices on his legs -- leave court a free man.

There's a simple question at the heart of the case: Did Pistorius know or suspect that his 29-year-old cover model girlfriend was in the bathroom when he fired four bullets through a locked door at her, killing her almost instantly?

If the judge is convinced that Pistorius, 27, did know he was firing at his girlfriend, he'll be convicted of murder.

If the judge accepts the disabled Olympic star's defense that he thought he was shooting at an intruder, she could convict him of the lesser charge of culpable homicide, or find him not guilty.

Was it in self-defense?

From the first phone call he made to a neighbor minutes after shooting Steenkamp in the early hours of February 14, 2013, Pistorius has maintained that he thought someone had broken into his house and that he believed he was firing in self-defense.

Nothing has shaken his story, not even a week of brutal cross-examination on the stand by state prosecutor Gerrie Nel, who barked at Pistorius: "Say it! Say 'I killed Reeva Steenkamp!'"

The pugnacious Nel, one of South Africa's most tenacious prosecutors, demanded that Pistorius look at a photo of Steenkamp's bloodied head after one of the athlete's bullets passed through her brain.

Weeping, his voice breaking, Pistorius refused, crying back: "I don't have to look at a picture! I was there!"

Pistorius' extreme distress -- which included repeated bouts of vomiting during some parts of the trial -- may have grabbed headlines and split opinions, but it's not clear that they will sway Masipa.

Colleagues described her to CNN as fair-minded and focused on the facts, not on emotions or the global media coverage of the case.

But undisputed facts are hard to come by when only two people were in Pistorius' house in a gated community on the outskirts of Johannesburg on the night of the killing -- the killer and the victim.

Neighbors' testimony questioned

Prosecution and defense attorneys spent months battling over what neighbors heard that night, when Pistorius last used his phone and when Steenkamp last ate, and whether the amputation of both of the runner's legs below the knee means that what is reasonable for an able-bodied person might not be reasonable for him.

A handful of neighbors testified they heard shouting or screaming on the night, as well as thuds or bangs which they thought were gunshots.

Nel wove that testimony into a tale of a heated argument between lovers, culminating in Steenkamp fleeing from her boyfriend and locking herself in the toilet before Pistorius shot her in a rage.

As the prosecution case came to an end in late March, Nel produced text messages between the two, including one in which Steenkamp said she was sometimes scared of Pistorius.

But lead defense lawyer Barry Roux chipped away at that story, producing phone records to cast doubt on the idea that the two fought the night Steenkamp died.

What neighbors heard was not a fight and a hot-blooded murder, Roux argued, but Pistorius shouting for help after he realized what he had done by mistake.

The alleged gunshots after the shouting were in fact the sounds of Pistorius trying to break down the toilet door to save Steenkamp, Roux argued.

Was Reeva afraid of her boyfriend?

And he downplayed the significance of the WhatsApp text messages suggesting Steenkamp feared Pistorius. There were only four such messages out of more than 1,700 the two exchanged in the four months they were dating, he argued.

Far more common were affectionate messages peppered with nicknames and "kiss" symbols, which he had a police witness read aloud in court in one of the few lighter moments in the emotionally wrenching six-month trial.

Each side produced its own pathology experts to try to establish when Steenkamp last ate -- a fact which could potentially poke a hole in Pistorius' story that the two went to sleep around 10 p.m., roughly five hours before he killed her.

The state said her stomach contents suggested she had eaten several hours later than that, while the defense said it was impossible to pinpoint with such accuracy when Steenkamp's last meal was and cast doubt on the likelihood of an athlete and a model eating dinner after midnight.

Psychology played a part

The defense outlined the psychological effects on Pistorius of his amputation before he was a year old, his parents' divorce, and his mother's death when he was a schoolboy.

It also said the psychological portrait showed Pistorius was a man temperamentally given to running toward danger, not away from it, and that he was physically unable to flee danger in a fight-or-flight scenario.

If the judge is not convinced beyond a reasonable doubt of the state's case that Pistorius knew he was shooting at Steenkamp, she must decide if his mistakes were genuine and reasonable.

If she rules they were not reasonable, she could find him guilty of culpable homicide. There's no mandatory sentencing guidelines for that charge, which is similar to manslaughter in the United States.

If she believes the runner made reasonable mistakes in the circumstances of that night, she could find him not guilty and set him free.

There is no jury. South Africa abolished the use of juries in 1969, during the days of apartheid, fearing that racism would taint jurors' decisions.

Not only a murder charge

Pistorius also faces three lesser weapons charges.

Two concern allegations that he recklessly fired a gun in a public place -- once at the popular Tasha's restaurant in Johannesburg, and once from the sunroof of a car when driving with his previous girlfriend and another friend.

Pistorius denies guilt in both cases. If he is found guilty on those counts, he could potentially face up to five years in prison for each charge, although a lesser sentence such as a fine or loss of his gun license is also possible.

The final charge is that he had in his house ammunition for which he did not have the proper license. He says he was storing it in a safe for his father and denies guilt.

It's a serious charge, potentially carrying a prison sentence of up to 15 years, but in that case, too, lesser punishments are possible.

Pistorius can request an appeal if he is found guilty of any of the charges. The state can also request an appeal if he is not, although with greater restrictions that a defendant's right to appeal.