Oscar Pistorius shot and killed Reeva Steenkamp after attaching his prosthetic legs and walking seven meters to the bathroom where she was hiding after an argument, prosecutors in South Africa argued Tuesday.
Pistorius later disputed the prosecution's claims, saying he believed Steenkamp was a robber when he shot and killed her.
Steenkamp, 29, was a model and a law school graduate who died at Pistorius' home in Pretoria on Valentine's Day. He appeared at a bail hearing in front of chief magistrate Desmond Nair on Tuesday where details of the killing were given by the prosecution and defense teams. The hearing was scheduled to last two days.
Nair ruled that Pistorius would be prosecuted on a charge of premeditated murder.
Pistorius released a statement, according to CNN, "I fail to understand how I could be charged with murder as I had no intention to kill my girlfriend."
"We were deeply in love," an affidavit for Pistorius said, according to The Telegraph, the British publication that was among several media outlets represented in the courtroom.
Pistorius said he had received death threats, so he kept a 9 mm pistol under his bed, The Telegraph reported.
Pistorius says he shot Steenkamp by mistake, believing she was a robber. He said he felt vulnerable because he did not have on his prosthetics when he shot through the bathroom door, the Associated Press reported.
When he realized Steenkamp was not in his bed, "It filled me with horror and fear ..."
The affidavit said Pistorius then put on his prosthetic legs, broke the door with a cricket bat to find Steenkamp inside the bathroom.
"She died in my arms," Pistorius' affidavit said. He said he ran downstairs with her.
Wearing a gray suit and tie, Pistorius broke down and sobbed at times during the hearing, the Associated Press reported. The magistrate told Pistorius he must try to compose himself.
News of Pistorius' arrest has shocked South Africa after his story captured the world's attention during last summer's London Olympics and Paralympics. The 26-year-old became the first double amputee sprinter to compete at the Olympics and went on to win two gold medals at the Paralympics weeks later.
Steenkamp had gone to her boyfriend's home to spend the night with him on Feb. 14, the prosecution said. They say the couple had an argument before she locked herself in the bathroom. Pistorius fired four shots, and Steenkamp was hit three times, the prosecution said.
Prosecutor Gerrie Nel argued Pistorius committed premeditated murder, while the defense said it was an accident.
"If I arm myself, walk a distance and murder a person, that is premeditated," Nel said, according to Reuters. "The door is closed. There is no doubt. I walk seven meters and I kill."
"The motive is 'I want to kill.' That's it."
Defense lawyer Barry Roux disputed the murder charge. "All we really know is she locked herself behind the toilet door and she was shot," he said.
Pistorius told his sister, Aimee, that he thought a burglar was in his house, the prosecution said.
While Pistorius appeared in court, Steenkamp's family held a private memorial service in Port Elizabeth, a southern coastal town about 700 miles from Pretoria. A day earlier, June Steenkamp, Reeva's mother, said the family sought answers.
"Why? Why my little girl? Why did this happen? Why did he do this?" she said in an interview published Monday in The Times of Johannesburg.
Pistorius' family has opposed the premeditated charge.
"After consulting with legal representatives, we deeply regret the allegation of premeditated murder," Arnold Pistorius, Oscar's uncle, told the Associated Press and two other South African journalists. "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 murder as such."
Known as the "Blade Runner" for his carbon-fiber running blades, Pistorius has had sponsorship deals with global companies such as Nike and Oakley.
Oakley released a statement Monday saying that it is suspending its contract with Pistorius, the Associated Press reported. Nike spokesman KeJuan Wilkins told the Associated Press that the shoe company had no plans to use Pistorius in future ads. He did not say whether Nike previously had plans for Pistorius or whether it pulled current ads featuring him.
Roxanna Scott, USA TODAY Sports
Contributing: Associated Press