Reeva Steenkamp, a model who spoke out on Twitter against rape and abuse of women, was shot four times in the predawn hours in the house, in a gated community in the capital, Pretoria, police said.
Hours later after undergoing police questioning, Pistorius left a police station accompanied by officers. He looked down as photographers snapped pictures, the hood on his gray workout jacket pulled up, covering most of his face. His court hearing was originally scheduled for Thursday afternoon but has been postponed until Friday to give forensic investigators time to carry out their work, said Medupe Simasiku, a spokesman for the prosecution.
South Africans were shocked at the killing. The Star newspaper printed a special afternoon edition Thursday that hawkers sold in the streets of Johannesburg, carrying the headline: "OSCAR ARRESTED: GIRLFRIEND KILLED."
While Pistorius captured the nation's attention with his Olympic dreams, police said there was a history of problems involving him.
There have "previously been incidents at the home of Mr. Oscar Pistorius," said police spokeswoman Brigadier Denise Beukes. Police in South Africa do not name suspects in crimes until they have appeared in court but Beukes said that the 26-year-old Pistorius was at his home at the time of the death of Steenkamp and "there is no other suspect involved."
"Yes, there are witnesses and there have also been interviews this morning," Beukes told reporters outside the gated complex where Pistorius lived. "We are talking about neighbors and people that heard things that happened earlier in the evening and when the shooting took place."
Pistorius' father, Henke, declined to comment when contacted by The Associated Press, only saying "we all pray for guidance and strength for Oscar and the lady's parents."
Neither Pistorius' agent Peet van Zyl nor coach Ampie Louw could be reached while Pistorius' own cellphone went straight to an answerphone service.
Pistorius' former coach, Andrea Giannini, said he hopes it was "just a tragic accident."
"No matter how bad the situation was, Oscar always stayed calm and positive," Giannini told the AP in Italy. "Whenever he was tired or nervous he was still extremely nice to people. I never saw him violent."
Gianni said he believed that Pistorius had been dating Steenkamp for "a few months."
"I know he had more than one flirt over the last year," Giannini said.
Paolo Urbani, the mayor of the Italian town of Gemona where Pistorius had a training base and prepared for the London Olympics from said he is shocked as Pistorius was "a delightful person."
"The news shocked not only me personally but also the whole of Gemona and the region," Urbani said. "It come(s) as a huge shock to everyone who knew him. I was woken up this morning by a phone call from his general manager, who called me to let me know so that I didn't find out about it from the news."
Police said that earlier reports that Steenkamp may have been mistaken for a burglar by Pistorius did not come from the police. Several local media outlets had initially reported that the shooting may have been accidental.
"It would be very premature and very irresponsible of me to say what actually has happened," Beukes said. "There have been allegations. We are not sure."
Beukes also said there had been previous incidents and "allegations of a domestic nature" at the home of the Olympic star and double-amputee runner, who is one of South Africa's and the world's most famous sportsmen and made history at the London Games last year by being the first double-amputee runner to compete at the Olympics.
"I'm not going to elaborate on it but there have been incidents (at Pistorius' home)," Beukes said.
Capacity Relations, a talent management firm, earlier named model Steenkamp as the victim of the shooting. Police spokeswoman Lt. Col. Katlego Mogale told the AP that officers received a call around 3 a.m. after the shooting.
A 9 mm pistol was recovered and a murder case opened against Pistorius.
Pistorius enjoyed target shooting with his pistol and an online advertisement featuring him for Nike read: "I am a bullet in the chamber." An article in January 2012 in The New York Times Magazine described him talking about how he pulled a pistol to search his home when his alarm went off the night before an interview. At Pistorius' suggestion, he and the journalist went to a nearby target range where they fired at targets with a 9 mm pistol. At one point, Pistorius told the writer: "If you practiced, I think you could be pretty deadly."
Asked how often he went target shooting, Pistorius replied: "Just sometimes when I can't sleep."
On Thursday, Mogale said when police arrived at Pistorius' house they found paramedics trying to revive a 30-year-old woman, who had been shot four times. Mogale, who was speaking to the AP from the scene, said the woman died at the house.
Police have still not released the name of the woman, but the publicist for Steenkamp confirmed in a statement that the model was dead.
"We can confirm that Reeva Steenkamp has passed away," Steenkamp's publicist Sarit Tomlinson said. "We are in communication with people on the scene, please wait for official statements, as there is too much speculation at this moment in time. We will provide further information as soon as we are able to provide accurate information as to what transpired.
"Our thoughts and prayers go to the Steenkamp family, who have asked to have their privacy respected during this difficult time, everyone is simply devastated. She was the kindest, sweetest human being; an angel on earth and will be sorely missed."
Tomlinson said Steenkamp, known simply as Reeva, was one of FHM's (formerly For Him Magazine) 100 Sexiest Women in the World for two years running, appeared in countless international and national advertisements and was one of the celebrity contestants on Tropika Island of Treasure, filmed in Jamaica.
On Twitter, she tweeted messages urging women to stand up against rape alongside her excitement about Valentine's Day. "What do you have up your sleeve for your love tomorrow?" she tweeted. "It should be a day of love for everyone."
Mogale and Beukes said the victim's family had not yet identified the body.
Pistorius made history in London last year when he became the first double-amputee track athlete to compete in the Olympic Games, propelling him to the status of an athletics superstar.
Having had both his legs amputated below the knee before his first birthday because of a congenital condition, he campaigned for years to be allowed to compete against able-bodied athletes. Having initially been banned because of his carbon fiber blades - which critics said gave him an unfair advantage - he was cleared by sport's highest court in 2008 and allowed to run at the top events.
He competed in the 400 meters and on South Africa's 4x400 relay team at the London Games, making history after having his selection confirmed on South Africa's team at the very last minute. He also retained his Paralympic title in the 400 meters in London.
South Africa's Sports Confederation and Olympic committee released a statement on Thursday saying they had been "inundated" with requests for comment but were not in a position to give out any details of the shooting.
"SASCOC, like the rest of the public, knows no more than what is in the public domain, which is there has been an alleged fatal shooting on the basis of a mistaken identity and an apparent assumption of a burglary," the South African Olympic committee said. "The organization is in no position to comment on the incident other than to say our deepest sympathy and condolences have been expressed to the families of all concerned."
The International Paralympic Committee also said it wouldn't comment in detail apart from offering its condolences to the victim's family.
"This is a police matter, with a formal investigation currently underway," the IPC said. "Therefore it would be inappropriate for the IPC to comment on this incident until the official police process has concluded. The IPC would like to offer its deepest sympathy and condolences to all families involved in this case."
South Africa has some of the world's highest murder rates, with nearly 50 people killed each day in the nation of 50 million. It also has high rates of rape, other assaults, robbery and carjackings.
U.N. statistics show South Africa has the second highest rate of shooting deaths in the world, second only to Colombia.
Imray reported from Cape Town, South Africa. Associated Press writer Michelle Faul contributed to this report from Johannesburg; AP Sports Writer Andrew Dampf in Rome contributed to this report.