South African double-amputee Paralympian star Oscar Pistorius went on trial Monday, accused of murdering his girlfriend Reeva Steenkamp, a law graduate and model.
As his lawyers launched an aggressive defence on day one of the trial, tearing into a witness who claimed to have heard "bloodcurdling screams" as his girlfriend was killed, three questions could be key to the trial:
Did the couple argue on the night of the shooting?
In his affidavit, Pistorius claimed the couple spent a quiet night at home. They ate dinner then Pistorius watched television while Steenkamp did yoga, before both fell asleep.
The prosecution is expected to challenge that account, calling on the testimony of neighbours who claim to have heard shouts and screams coming from the couple's house.
The couple's mobile phone records could also offer evidence of a fight. Steenkamp is said to have had her phone with her in the toilet when she was shot. Did she contact friends?
Read: Pistorius pleads not guilty at murder trial
Authorities have been trying to get access to Pistorius's iPhone. He claims to have given them the correct password, but it did not work. Does it contain evidence of a fight in the form of text messages, or as one local paper recently reported, evidence that Pistorius was looking at porn on the night of her death.
If the couple did in fact argue early that Valentine's Day it would contradict Pistorius's account of what he calls "a tragic accident" and provide the prosecution with a motive for premeditated murder.
Did the police irreparably taint the crime scene?
In the bail hearing, then lead detective Hilton Botha admitted he had entered Pistorius's home and investigated the scene without the correct protective footwear, lost track of ammunition at the scene and made several false assumptions.
In a case where there is only one surviving witness -- the accused -- forensic and material evidence is expected to be key. The treatment of the crime scene may throw the reliability of that evidence into doubt.
Read: Pistorius defence tears into first witness
Already the prosecution looks set to concede that Pistorius was further away from the door when he shot than they first argued and that he was not in fact wearing his prostheses.
Both arguments were initially put forward as evidence that Pistorius was not in fact scared and his actions were premeditated.
Why did Pistorius not call the police?
In his affidavit, the sporting hero known as the "Blade Runner" for his carbon-fibre running blades, says he called two numbers after realising he shot Steenkamp: Johan Stander, who was involved in the administration of the gated community where he lived and private medical service Netcare.
The prosecution is expected to probe his relationship with Stander and why Pistorius allegedly dismissed security guards who rang his house after hearing gun shots, telling them everything was fine.
The prosecution could attempt to prove Pistorius killed Steenkamp in a rage and then attempted to cover up evidence.