GRAHAM, Wash. (AP) -- Before setting his house ablaze and killing himself and his two young sons, Josh Powell left a voicemail for family members saying he couldn't live without the boys and didn't want to go on anymore.

ABC News obtained what it says was a voicemail Powell left for his family members. In the recording played Tuesday on "Good Morning America" Powell also said he was calling to say goodbye and apologize.

"I'm sorry to everyone I've hurt," he is heard saying. "Goodbye."

Powell, the husband of missing Utah woman Susan Powell, died along with his children Sunday in Washington. An autopsy showed the children also suffered hatchet wounds to their necks. He was a person of interest in his wife's 2009 disappearance.

As authorities work to determine exactly why Powell committed the murders at the Washington residence, investigators said they're no closer to answering the question they've had for two years.

"We still haven't identified or found where Susan Powell is," said West Valley, Utah, Police Chief Buzz Nielsen, who is in charge of the investigation into the young mother's disappearance.

The father of the 5- and 7-year-old boys painted himself as a tortured man, ridiculed without reason in the disappearance of his wife, steadfastly insisting he was innocent until the end.

Yet the investigation and autopsy reports released late Monday show the acts of a violent man who meticulously planned the double murder-suicide of himself and his two young sons, culminating Sunday in Powell setting his house ablaze and taking a hatchet to his children's necks.

For now, even with the longtime "person of interest" in the probe dead, police say the case remains open, though some mysteries may never be solved.

Powell's horrific murder-suicide seemed to come out of nowhere. Just days before, in a motion seeking custody of his children filed with a Washington state court, Powell said he missed his wife, and would remain strong for the boys.

"A lesser person would fall under the intense scrutiny I am facing, but apparently my inherent resilience as a person makes it increasingly difficult for them to pursue their agendas," Powell wrote. "I am standing tall for my sons, but it deeply hurts to face such ridicule and abuse.

"I know my own heart is free of any guilt regardless of what people claim," he added.

Things changed dramatically when the judge ruled against him, ordering the children to remain with Susan Powell's parents for now.

On Sunday, Powell's boys came for a routine supervised visit. They ran ahead, the social worker falling behind. Powell then locked the door, used a hatchet on his kids, and lit the house on fire.

Ultimately, Powell and both boys died of smoke inhalation, Pierce County Medical Examiner's Office investigator Melissa Baker said Monday evening.

But they also suffered "chop injuries" that contributed to their deaths - 7-year-old Charles was struck on his neck and 5-year-old Braden had injuries to both his head and neck, Baker said.

Pierce County Sheriff's Detective Ed Troyer said investigators found a hatchet that they believe was used on the boys.

"We recovered a hatchet - a small ax," he said. "It was right there with" the bodies.

Authorities also said Powell had made thorough plans well ahead of the murders.

"This was definitely a deliberate, planned-out event," Troyer said.

He said 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 on 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.

But, he added, "There's no indication about Susan in anything that we've found so far."

Nielsen said detectives want to question Powell's father, Steve Powell, about Susan's disappearance. He described Steve Powell as another "person of interest" but noted the elder Powell is "not in our sights" in terms of any potentially imminent arrest.

Steve Powell has been in jail on voyeurism and child porn charges since last fall after authorities found explicit images on his computers during a search of his home in the case of his missing daughter-in-law.

After his arrest, the state turned the boys over to Susan Powell's parents, Charles and Judy Cox.

Steve Powell claimed on national television last year to have had a flirtatious or even sexual relationship with Susan - something her family has adamantly denied.

Josh Powell claimed that the night his wife vanished in December 2009, he took the boys from their West Valley City home, about 10 miles outside Salt Lake City, on a midnight camping trip in freezing temperatures - a story her parents never believed. Authorities searched the area in the central Utah desert but came up empty.

Less than a month after the disappearance, Josh Powell moved the boys to his father's home in Puyallup, south of Seattle.

Sunday's tragedy left the Coxes devastated. They spoke to reporters Monday to give a glimpse of the lives the boys led.

They said the boys played happily and didn't want to visit their father when the time came for their weekly Sunday visit. But Judy Cox said she talked them into going - and she now regrets it.

Charles Cox said he didn't necessarily think there was any more the court could have done legally to protect his grandchildren. However, he said he didn't like that there was only one supervisor during their visits with their father.

"We suspected that if he had the boys in his control, with him, and he felt the police were closing in, he was capable (of hurting them)," Cox said. "We didn't like that there was only one supervisor. Frankly, she couldn't have stopped him if he wanted to do something."

The boys were emotionally distant when they first arrived at their grandparents' home, Charles Cox said, but recently they had become warmer. And that gave the grandparents hope that maybe someday they would be able to relate what happened to their mother.

"They were like little robots. If you asked them about mommy, they would run away," he said. But "in the last week, I could not sit down without them climbing up on my lap."

Charles Cox said that the summer after his daughter disappeared, Braden drew a picture at day care of a van with three people in it, and told caregivers who asked him about it that it was a picture of his family going camping: "Mommy's in the trunk," the boy reportedly said.

But the boys had not recently made more comments to that effect, Charles Cox said, contradicting earlier remarks by his lawyer, Steve Downing, who told The Associated Press that the older child had recently mentioned their mother being in the trunk.

Nielsen said Utah authorities would continue with their investigation, and hoped to make an arrest in the Susan Powell case this year.

"On a criminal case of this nature, you've got one shot. You've got to make sure everything is done right," Nielsen said. "Our case is not closed."

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Brian Skoloff reported from Salt Lake City, Utah. Reporter Gene Johnson contributed from Seattle.

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