Sheriff's deputies arrived at the scene and found only Johnson and Klein. Johnson was hysterical and covered in blood. Klein was lying on a couch in Johnson's living room, unconscious and bleeding. Klein and Johnson each had been shot twice. Johnson recovered; Klein died of his wounds. Johnson said after falling asleep, he awoke to find Richter (D) and Branscombe in his bedroom, at which point Branscombe shot him. Johnson heard more gunfire in the living room and the sound of his assailants leaving. He got up, found Klein bleeding on the living room couch, and called 911. A gun safe, a pistol, and $6,000 cash, all of which had been in the bedroom, were missing. Evidence at the scene corroborated Johnson's account. Pools of blood had collected in the kitchen and the doorway to Johnson's bedroom. Investigators took only a few blood samples from the crime scene. One was from a blood splash on the wall near the bedroom doorway, but no sample was taken from the doorway blood pool itself. Investigators searched D's residence and found Johnson's gun safe, two boxes of .22-caliber ammunition, and a gun magazine loaded with cartridges of the same brand and type as the boxes. A ballistics expert later concluded the .22 bullet that struck Klein and the.22 shell found in the living room matched the ammunition found in D's home and bore markings consistent with the model of gun for which the magazine was designed. D and Branscombe were arrested. D denied involvement. He would later admit taking Johnson's pistol and disposing of it and of the .32-caliber weapon Branscombe used to shoot Johnson and Klein. D's counsel produced Johnson's missing pistol, but neither of the guns used to shoot Johnson and Klein was found. Branscombe and D were tried together on charges of murder, attempted murder, burglary, and robbery. Only Richter's case is presented here. The prosecution built its case on Johnson's testimony and on circumstantial evidence. D offered explanations for the circumstantial evidence and derided Johnson as a drug dealer, a paranoid, and a trigger-happy gun fanatic who had drawn a pistol on Branscombe and D the last time he had seen them. And there were inconsistencies in Johnson's story. In his 911 call, Johnson first said there were four or five men who had broken into his house, not two; and in the call, he did not identify D and Branscombe among the intruders. Investigators had not analyzed the few blood samples taken from the crime scene. D's attorney outlined the theory that Branscombe had fired on Johnson in self-defense and that Klein had been killed not on the living room couch but in the crossfire in the bedroom doorway. Defense counsel stressed deficiencies in the investigation, including the absence of forensic support for the prosecution's version of events. Without advance notice and over the objection of D's attorney, one of the detectives who investigated the shootings testified for the prosecution as an expert in blood pattern evidence. He concluded it was unlikely Klein had been shot outside the living room and then moved to the couch, given the patterns of blood on Klein's face, as well as other evidence including 'high velocity' blood spatter near the couch consistent with the location of a shooting. A serologist testified the blood sample taken near the pool by the bedroom door could be Johnson's but not Klein's. The detective who testified on blood patterns acknowledged that his inferences were imprecise, that it was unlikely Klein had been lying down on the couch when shot, and that he could not say the blood in the living room was from either of Klein's wounds. The serologist admitted she had not tested the bedroom blood sample for cross-contamination. She said that if the year-old sample had degraded, it would be difficult to tell whether blood of Klein's type was also present in the sample. D testified he and Branscombe returned to Johnson's house just before the shootings in order to deliver something to one of Johnson's roommates. Branscombe entered the house alone while D waited in the driveway; but after hearing screams and gunshots, D followed inside. There he saw Klein lying not on the couch but in the bedroom doorway, with Johnson on the bed and Branscombe standing in the middle of the room. Branscombe said he shot at Johnson and Klein after they attacked him. D's former girlfriend said she saw the gun safe at D's house shortly before the shootings. D was sentenced to life without parole. On appeal, his conviction was affirmed. The California Supreme Court denied a petition for review. D later petitioned the California Supreme Court for a writ of habeas corpus. He asserted a number of grounds for relief, including ineffective assistance of counsel. D claimed his counsel was deficient for failing to present expert testimony on serology, pathology, and blood spatter patterns, testimony that he argued, would disclose the source of the blood pool in the bedroom doorway. D offered affidavits from three types of forensic experts. There was a possibility Klein's blood was intermixed with blood of Johnson's type in the sample taken from near the pool in the bedroom doorway. The blood pool was too large to have come from Johnson given the nature of his wounds and his own account of his actions while waiting for the police. He provided a statement from an expert in bloodstain analysis who said the absence of 'a large number of satellite droplets' in photographs of the area around the blood in the bedroom doorway was inconsistent with the blood pool coming from Johnson as he stood in the doorway. The California Supreme Court denied D's petition. D filed a petition for habeas corpus in the District Court. It denied his petition, and a three-judge panel of the Court of Appeals for the Ninth Circuit affirmed. The Court of Appeals granted rehearing en banc and reversed the District Court's decision. The Supreme Court granted certiorari.