PRETORIA, South Africa - Oscar Pistorius once said he mistook the sound of a washing machine for an intruder and went into "combat mode" in his house, a South African guns expert testified Monday at the athlete's murder trial. Months later, Pistorius killed his girlfriend by shooting her through a closed toilet door. Sean Rens also said the Olympian had a "great love and enthusiasm" for guns and wanted to buy seven more in 2012, to add to the 9 mm pistol he already owned. He asked Rens, a firearm trainer, to procure them for him under a gun collector's permit.The guns included a semi-automatic assault rifle, three shotguns, another self-loading rifle and two Smith & Wesson handguns. Rens, the manager of the International Firearm Training Academy in Walkerville, south of Johannesburg, produced one invoice that included five of the guns and nearly 600 hundred rounds of ammunition made out in Pistorius' name.That total price came to nearly $5,000 and Pistorius had already paid about $4,500 of the bill, the invoice showed.Pistorius said he killed Steenkamp by accident, mistaking her for an intruder in his home.Rens testified that he met the double-amputee runner in 2012 and trained with him at a gun range. In one conversation they had about guns, Pistorius described how he was startled by a noise at home and decided to clear the house by drawing his gun and checking rooms.Pistorius tweeted about the incident in November 2012: "Nothing like getting home to hear the washing machine on and thinking its (sic) an intruder to go into full combat recon mode into the pantry!"Prosecutor Gerrie Nel questioned Rens on Pistorius' firearm training. He asked the dealer to describe how Pistorius was quizzed on how to handle a firearm in various scenarios, for example when two unidentified men approach the house of a gun owner; then when they break into the house, begin to steal belongings and order the gun owner to leave; and if the men threaten to kill the gun owner, who is behind a security gate in the house.In each case, Rens said, Pistorius was asked if it was OK to fire at the men and Pistorius correctly answered "No." Pistorius correctly noted he was entitled to shoot at them only if they advanced on him with a gun, according to Rens.Nel was trying to show that Pistorius knowingly flouted the regulations when he shot Steenkamp. Pistorius says he fired because he thought he was under threat.Later Monday, police photographer Bennie van Staden testified about the images he took at Pistorius' house on the night of the killing. One photograph of the runner showed unexplained scuff marks on his bloodied prosthetic limbs.Another from the Paralympian's bedroom showed a box with a label that said "Testis compositum."The runner's representatives have identified the substance as an herbal remedy used for "muscle recovery." A product by that name also is sold as a sexual enhancer and contains the testicles, heart and embryo of pigs, among other ingredients. Some retailers also say it can be used to treat fatigue.Also found in the bedroom was a box with the label "Coenzyme compositum" as well as syringes and needles.Steenkamp's mother, June, was in the Pretoria courtroom Monday but had left by the time van Staden was describing some photos being displayed of the bloody bathroom scene where her daughter was shot. June Steenkamp last attended the trial on the opening day, March 3.