(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