This is why it's a great idea to try these cases on Hacker News. (I'm only half kidding.) Always and everywhere, if you sit on a jury and listen only to the prosecutor or defense attorney, you will come to the conclusion they want. This is for the same reason you can't tell what a magician is doing with his hands: he's an pro, you're an amateur.
From:
http://www.cjlf.org/deathpenalty/CooperReview.htm
3. Josh Ryen told the police he thought three men committed the attack. He later changed his story.
"When Josh was rescued the day after the murders, he could not talk because his throat had been slashed. He could only squeeze the police officer's hand in response to questions. The story that Josh was finally able to tell police was that he was awakened in the middle of the night by his mother's screams. When he and his friend Chris went to investigate, he saw the bodies of his parents and Jessica and the backside of one unfamiliar person, so he ran and hid. Then he heard Chris screaming, so Josh ran back towards his friend. At that point, something struck him in the head, knocking him unconscious. He awoke later in a pool of blood.
When later queried by investigators, Josh spoke of three Mexicans who had come to the house earlier and thought they could have done it because they had been there once before. However, Josh never said he saw three people commit the murders. He consistently told different investigators that he saw only one attacker. The triple murderer theory is merely speculation based on the visit of the three Mexicans and twisting of a little boy's words.
Additionally, Josh was an eight-year-old boy who was startled awake by a horrific murder and was brutally attacked. It is unsurprising that probing questions by adults and the power of suggestion later tried to confuse his story. Most important however, Cooper was not convicted on the limited testimony of an eight-year-old. He was convicted by the mountain of other evidence incriminating him."
1. The girlfriend of a former inmate friend of Cooper's, thought her boyfriend might have been involved in the murder. She turned his bloody coveralls over to the local Yucaipa sheriff's substation, but they threw out the coveralls without testing them.
"This girlfriend, Diane Roper, was dismissed by law enforcement as completely lacking credibility. She was a professed witch who claimed she had a vision during a trance that the murder had been committed after she heard about the Ryen case. However, she had no substantive reason to believe her boyfriend was involved with Cooper the night of the murder. In fact, she told sheriff's investigators that she did not even know to whom the coveralls belonged. She said she "just knew" from the vision that the coveralls were connected to the case. By the time the San Bernardino County Sheriff's Department heard about her fantastic story, they had Cooper in custody with mountains of evidence (see above) against him. Based on their limited resources and already having the likely killer in custody, the San Bernardino County police chose not to expend precious time and money chasing Roper's crazy story."
As for the other evidence:
"The victims died from numerous chopping wounds later determined to have been inflicted by a hatchet or axe and stabbing wounds inflicted by both a knife and an ice pick. Later that day, bloodstained items were found in the vacant house where Cooper had stayed, including a button from a prison jacket identical to the one he was wearing when he escaped. A police criminologist also found evidence of blood on the carpet, in the bathroom sink and in the shower along with Cooper's footprint. Hairs from the shower drain and the bathroom sink were consistent with those from two of the victims.
A bloodstained hatchet from the vacant house was later found near the Ryen home. The sheath from the hatchet was found on the floor of the bedroom where Cooper had slept. Some hunting knives and at least one ice pick were also missing from the vacant house. A strap fitting one of the missing knives was found in the same bedroom. Shoe prints were found in the Ryen home and the vacant house next door matching the unique pattern of shoes issued exclusively to prison inmates. The prints indicated shoes of Cooper's size and brand that he had recently received in prison.
While most of the blood samples taken at the murder scene were determined to have come from the victims, one sample was conclusively determined to have come from a black person with the same blood group as Cooper. The sample was too small to determine if it was Cooper's rare blood type.
The Ryen station wagon was found several days after the killings in a church parking lot in Long Beach. Hairs found in the car matched those of Cooper. Tobacco issued exclusively to prison inmates, which Cooper smoked, was found in the vacant house and in the Ryen's station wagon.
Two days after the murders, Cooper befriended a couple in Mexico and joined them on a boat trip up the California Coast. Weeks later, Cooper was arrested on a boat off of Santa Barbara after the woman reported that he had raped her at knife point, threatening to kill her if she woke her husband. Following his arrest, several items taken from the vacant house in Chino were discovered on the boat.
At his trial, Cooper admitted staying in the Chino house but denied any involvement in the Ryen murders. Josh Ryen, who miraculously survived his injuries, testified that he awoke on the night of the murders after hearing his mother's screams. He remembered being hit from behind when trying to investigate but was unable to identify his attacker.
For the 19 years following his 1985 conviction, Cooper's claims of trial and sentencing errors have been reviewed by California and federal courts. In 2000 he won a delay of his execution so that new DNA testing could be performed on various blood and saliva samples found at the murder scene, in the stolen station wagon, and on a bloody t-shirt found near the Ryen home. The DNA from all of these samples was found to have come from the same person. This DNA was then compared to DNA from Cooper's blood. It matched. The odds of the match being by chance were 1 in 310 billion."
When you hear one side of the story, always look for the other -- whether or not it's a criminal trial. Adversarial justice works.
EDIT: show quotes clearly.