Released Monday evening, the autopsy results showed that all three members of the family were killed by carbon monoxide poisoning.
While it wasn't the official cause-of death, the medical examiner's report showed that both boys had significant "chopping injuries" to the head and neck.
Pierce County sheriff's spokesman Ed Troyer said deputies recovered a hatchet from the burned out rubble where Powell's house once stood.
"We recovered a hatchet - a small ax," he said. "It was right there with them (the bodies)."
Investigators now believe Josh Powell attempted to murder the children with the hatchet, but was overwhelmed by smoke before he could finish the job
Troyer said the investigation is now over.
"I hope there's nothing else to say about this, because we've pretty much hit bottom with this cause of death," he said. "Let's hope it doesn't get any worse."
The Utah police chief heading the investigation into Susan Powell's disappearance, Buzz Nielsen of the West Valley City Police Department, flew to Washington state Monday and vowed that the case will remain open.
Detectives hope to interview the father of Josh Powell, Steve Powell, who remains in jail on child pornography and voyeurism charges, Nielsen said. He identified Steve Powell as a "person of interest" in the disappearance but then said no arrest was imminent: "He's not in our sights."
Josh Powell had long been identified as a person of interest in his wife's disappearance, and last week a judge denied another request for his boys to be returned to him.
Powell claimed that the night his wife vanished in December 2009, he took the boys on a midnight camping trip in freezing temperatures - a story neither her parents nor police believe.
Asked why Josh Powell had never been arrested, Nielsen said: "In a criminal case of this nature, you've got one shot. You've got to make sure everything is done right." He said investigators were making progress and had hoped to make an arrest within the year, but declined to discuss what evidence they might have had against Josh Powell.
If Powell did know what happened to his wife, he appears to have kept that secret when he died.
Troyer said that minutes before the fire, Powell sent emails to several people saying, "I'm sorry. Goodbye." To others, including his cousins and pastor, he sent longer emails, with instructions such as where to find his money and how to shut off his utilities.
In at least one email, he wrote that he couldn't live without his boys, Troyer said.