He had shot his girlfriend about an hour earlier.
Pistorius was a celebrated track runner who rose from the hardship of having his legs amputated as a baby to compete at the London Olympics. He now faces 25 years to life in prison if convicted of premeditated murder.
The prosecution says Pistorius, 27, killed Steenkamp, 29, intentionally after a loud fight. The athlete maintains he shot her with his licensed 9 mm pistol by mistake, thinking she was an intruder in his bathroom.
While prosecutors have recreated the bloody crime scene and Pistorius' physical condition through a sequence of photographs over two days in court, Pistorius' chief defense lawyer, Barry Roux, embarked on a minute examination of what he contends is major police bungling in the hours, days and even months after the shooting.
Roux said that officers moved evidence and disturbed the scene at the home, and failed to wear proper forensic clothing while picking through it. Police concede they kept the most crucial object — the wooden toilet cubicle door through which Pistorius shot Steenkamp — in a body bag in an office and that some of the fragments of wood disappeared.
Defense experts said they have also found key marks in the door that were not identified by police.
A former police colonel, one of the first on the scene, also testified Friday that he had to have police forensic experts frisked and their bags and cars searched after one of a collection of nine expensive watches in Pistorius' bedroom disappeared. G.S. van Rensburg said he opened a case of theft.
Roux's early questions Friday reflected one of the key arguments in Pistorius' defense — that police tampered with the scene and contaminated the evidence.
"Were you acutely aware that you should not disturb the scene? Did you have that awareness?" Roux asked van Rensburg, who said he arrived just before 4 a.m., about 30 to 40 minutes after prosecutors say Pistorius shot Steenkamp.
Roux honed in on small details. He said a small towel covering a cellphone in the bloody bathroom was moved so that it could be photographed, and that the evidence notes incorrectly presented the image as the crime scene as it was found. A fan, curtains and an overnight bag and a pair of sandals apparently belonging to Steenkamp also were moved without van Rensburg's knowing when and by whom, he said.
There were bigger concerns.
During the investigation in the bathroom, van Rensburg testified that he turned around at one point to see that the firearms expert was handling the silver and black handgun Pistorius used to kill Steenkamp and had taken the magazine out of the weapon without using gloves.
"I asked him, 'What are you doing?'" van Rensburg said. The officer realized his error, apologized, put the magazine back in the gun and put it back down, according to van Rensburg. He then put on his forensic gloves.
"I was very angry," van Rensburg said.
The prosecution also posed questions, however, through its collection of photos from the inside of Pistorius' house. The photos showed blood stains and smatter on walls, floors and furniture, both upstairs and downstairs, where Pistorius says he carried Steenkamp to get help. They also revealed dents in the bedroom door, broken tiles in the bathroom and a metal panel on the wall that had been bashed in.