(CNN) -- The deaths of a Washington man and his two sons in what authorities believe was a murder-suicide may mean the 2009 disappearance of the children's mother may never be solved.
Josh Powell, a suspect in the disappearance of Susan Cox-Powell, died Sunday along with his two sons, 5-year-old Braden and 7-year-old Charlie, in what police believe was an intentionally set fire in Powell's Puyallup, Washington, home.
It was a tragic development in a puzzling case that began two years ago in the Salt Lake City suburb of West Valley City, Utah, when Susan Cox-Powell, 28, went missing.
Josh Powell was never charged in her disappearance, and was embroiled in a bitter custody dispute with his wife's parents.
Now, with his death, Cox-Powell's relatives may never learn her fate.
The missing woman's parents "firmly believed that (Josh Powell) was clearly a person of interest in her disappearance and her death, and they were very much opposed to him having any contact with the children or custody," attorney Anne Bremner, who represented Charles and Judith Cox in the disappearance of their daughter, told CNN Sunday.
"Today is, of course, (a) devastating, horrific, unspeakable day," she said, but "... Chuck and Judy, my clients, had always had very real and very serious fears about Josh Powell."
Before Powell set fire to his home, he sent his attorney an e-mail saying simply: "I'm sorry. Goodbye."
"This was all on him," said Ed Troyer, spokesman for the Pierce County, Washington, Sheriff's Department. "He set this up. He did it."
The sheriff's department has copies of an e-mail Powell sent to his attorney as well as family and friends that said "he couldn't live with what was going on," Troyer said.
He said Sunday that the bodies had not been formally identified by a medical examiner, but "we believe it is the three of them."
"This was something that was done deliberately and intentionally, and the plan was carried out very quickly," he said.
The bodies were found together in the same room of the home, Troyer said.
The children, at least the older boy, Charlie, had reached the age where they may have been giving away information, Bremner said.
"I know that to be true in this case because as recently as Christmas, the boys told their grandparents, 'Mommy's in the mine. If we go to the mine, we'll find Mommy,'" she said.
Authorities have searched mines in Utah and Nevada for Cox-Powell, she said.
Powell was the sole suspect in Cox-Powell's disappearance, according to West Valley City Police Chief Buzz Nielsen.
"We always kept Josh as a target," Nielsen told CNN affiliate KSTU-TV.
According to investigators, Powell had said the last time he saw his wife was the night he and his sons -- then ages 2 and 4 -- left to go camping.
Powell told authorities he left the house sometime after midnight in below-freezing weather for a desert area in Utah's Tooele County.
Cox-Powell's sister eventually reported her missing. A month later, Powell and his children moved from Utah to Washington.
As late as last month, Utah authorities were still working to connect Powell to the disappearance of his wife, who authorities believe is dead.
"We just needed a little bit more. Everywhere we looked, it always came back to Josh," Nielsen said.
However, "I can tell you that Josh was terrorized," Powell's cousin, Nathan Leach, told The Salt Lake Tribune on Sunday. "I'm sure there are people out there who would consider this appropriate given the mass assumption of his guilt. We all want to see criminals pay for what they have done, but when evidence lacks for due process to make this happen, is this the appropriate method of alternate prosecution?"
"We are all outraged at what has happened, both for Josh's really abhorrent final act and for the actions of the other people involved. We can't help but wonder if this could have been prevented otherwise," Leach told the newspaper.
In a statement released by Bremner on Sunday, the Cox family asked for "time, privacy and prayers after today's horrific events."
West Valley City police said in a statement Sunday the investigation into Cox-Powell's disappearance continues. A team of detectives was sent to Washington in the aftermath of the fatal fire, authorities said.
The tragic events appeared to have been set in motion days earlier when a judge refused Powell's petition to regain custody of his children and instead ordered he undergo psychological evaluations -- an order that came after authorities turned up child pornography in the home Powell shared with his father.
As part of the investigation into Cox-Powell's disappearance, Utah authorities searched the Washington house where Powell, his two sons and his father, Steven Powell, were living last year.
During the search, investigators "discovered numerous images and recordings of adult and juvenile females," according to a statement released by the Pierce County sheriff's department.
Powell's father, Steven, was subsequently
charged with 14 counts of voyeurism and one count of possessing images of children engaged in sexually explicit conduct, according to court documents.
Following the arrest of Powell's father, custody of his sons was given to the Coxes, according to Washington state court records. Powell maintained in court documents filed last week that he established his own home after his father's arrest and "have consistently proven by fitness as a stable and loving parent under close supervision by (child welfare caseworkers)."
"The real story is not that anyone is a bad person, including me," Powell said in the affidavit. "The story is one of overcoming and rising above what many people think are insurmountable challenges."
On Sunday, shortly after noon, Powell was standing outside his home in a quiet, tree-lined cul-de-sac waiting for a social worker to bring the two boys for a supervised visit, authorities said.
As the children got to the door, Powell pushed the social worker back, quickly brought the two boys inside and locked the door.
The social worker, who later reported smelling something similar to gas at the time, tried "pounding the doors, trying to get in," said Gary Franz, a deputy chief with Graham, Washington, Fire and Rescue.
About two minutes later, as the social worker was calling her supervisor, the house exploded, Franz said.
The powerful explosion shook houses and rattled windows, with debris landing on lawns blocks away.
"All the walls in the house were on fire, almost immediately," neighbor Ryan Mickle told CNN affiliate KIRO-TV. "I didn't hear anybody inside."
Jennifer Bleakley, who lived near the Powell home, was making coffee in the kitchen when the explosion violently rattled windows and doors in her home.
"It really hurts my heart. My heart ached for those boys anyway," she told CNN affiliate KOMO-TV, wiping away tears.
News of the explosion and deaths quickly made it to Utah, sending shock waves through the Cox family.
"I think this is his admission of guilt, and he just couldn't handle it anymore," Kirk Graves, Powell's brother-in-law, told CNN affiliate KUTV-TV.
Grief, sadness and anger were expressed Sunday and Monday in a Facebook group, "Friends and Family of Susan Powell."
"We will never forget Susan, Charlie or Braden," wrote one group member. "May those close to this story know we all love them and will continue to pray that Susan is found."
-- CNN's Chelsea Carter, Greg Botelho, Ashley Hayes, Sara Weisfeldt, Leslie Tripp and Dave Alsup contributed to this report.
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