By LIEZL THOM

PRETORIA, South Africa - Oscar Pistorius spent much of last week on the stand detailing for the court what happened the night he shot and killed his lover Reeva Steenkamp.

His version was repeatedly challenged by prosecutor Gerrie Nel who called it "improbable" and a "lie."

Pistorius' credibility will likely play a key part when Judge Thokozile Masipa forms her verdict. If convicted of murder, Pistorius could be sentenced to at least 25 years in prison.

Here is how Pistorius claims he came to kill his girlfriend in the predawn hours of Valentine's Day 2013:

• Pistorius and Steenkamp went to bed about 10 p.m. Pistorius arranged two fans in the doorway to the balcony because the air conditioning wasn't working.
• Pistorius usually sleeps on right hand side of the bed but claims that due to shoulder injury he slept on left-hand side of bed that night. His iPad and prosthetic legs were on the right hand side of the bed. Steenkamp's bag and sandals were on the left-hand side, the side he claims he was sleeping on.
• He woke up during the early hours of Feb. 14, 2013 because the room was hot and humid. Steenkamp was also awake and asked him, "You can't sleep, Baba?"
• Pistorius got up without his "legs" on and moved the fans from the balcony doorway to inside the room. He pulled the curtains tight to block a slim bit of light through the curtains and moved Steenkamp's jeans from the floor and was going to use them to cover a small LED light on an amplifier.
• Before covering the light, he heard the noise of a window opening in the bathroom.
• He said he believed the noise was from an intruder who was in his house, possibly via a ladder to a window.
• Still on his stumps, Pistorius retrieved his gun from beneath the left side of the bed. He whispered to Steenkamp to call the police and moved into a hallway leading to the bathroom. The gun was pointed in front of him.

This image from Sky News shows Oscar Pistorius using the same pistol that killed his model girlfriend Reeva Steenkamp. Sky News

• Moving down the hallway, he leaned against the wall for better stability on his stumps. His eyes kept moving between the bathroom door and a nearby window.
• He yelled for the "person" to get out of his house and yelled to Steenkamp to call the police.
• He felt vulnerable on his stumps and feared someone was about to come out of the bathroom when he heard the sound of "wood moving" and thought the door was opening. He was "overcome with fear" and he fired four shots in quick succession through the bathroom door.
• His ears were ringing from the gunshots, so he was unable to hear anything.
• Pistorius retreated back down the hallway to the bed. Still aiming his gun at the bathroom, he hopped on the bed in the dark. He felt for Steenkamp and didn't find her. He thought maybe she was hiding on the floor or behind the curtains, so he checked the floor and ran his hands along the curtains. That was when he first suspected Steenkamp may have been in the bathroom, "when everything changed."
• Pistorius went back to the bathroom door, still holding the gun, and tried the doorknob. The door was locked.
• He went back to the bedroom, put on his legs, opened the curtains to the balcony while screaming for God to help him. With his legs on, he slammed into the bathroom door with his body. He got a cricket bat and smashed a hole in the door so that he could reach in, pick up a key from the floor and let himself in.

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  Forensic investigator Johannes Vermeulen is pictured with a cricket bat, demonstrating on a mock-up toilet and door details of how the door could have been broken down, in Pretoria, South Africa on March 12, 2014. Alexander Joe/AP Images

• Pistorius found Steenkamp crumpled on the floor with her on the toilet. He sat over her and cried.
• He ran to the left side of his bed to retrieve his phone and went back to Steenkamp. At 3:19 a.m. he called a neighbor. At 3:20 a.m. he phoned the ambulance service, and at 3:21 a.m. he called security at his estate.
• Pistorius picked up Steenkamp and took her downstairs near the front door. When a neighbor arrived, Pistorius had his fingers in her mouth to keep her airway open and a hand on her hip to staunch the bleeding.
• Police and paramedics arrived. Pistorius is asked to stay in the kitchen.
• A paramedic told Pistorius that Steenkamp was dead.
• Pistorius asked the police if he can wash his bloody hands because the smell was making him feel ill.
• About two hours later, Pistorius asked the police to take whatever photos of him that were needed so he could take off his blood soaked clothing.

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  An image taken from the court pool TV via AP showing on screen a police photograph of Oscar Pistorius standing on his blood-stained prosthetic legs and wearing shorts covered in blood, taken shortly after the athlete fatally shot his girlfriend, which was shown to the court in Pretoria, at his murder trial March 14, 2014. AP Photo/Court Pool via AP