Puyallup, Washington (CNN) -- The two sons of a Washington man killed along with their father in a weekend fire suffered hatchet wounds before dying of carbon monoxide poisoning, authorities said Monday.
Investigators found a hatchet in the home of Josh Powell, whom authorities believe set the Sunday afternoon fire, said Ed Troyer, a spokesman for the Pierce County, Washington, Sheriff's Department. Powell was a suspect in the 2009 disappearance of his wife, Susan Cox-Powell.
An autopsy showed his sons, 5-year-old Braden and 7-year-old Charlie, suffered chop injuries to their necks, but medical examiners concluded both boys and their father died from inhaling carbon monoxide, the county medical examiner's office reported.
Powell's death was ruled a suicide, while his sons' deaths were determined to be homicides. Powell first gave toys and books to charity, sent multiple goodbye e-mails and doused the Puyallup, Washington, home with gasoline before setting it ablaze, authorities said Monday.
That evidence suggests that Josh Powell planned a murder-suicide for some time, Troyer said.
"He was making arrangement to leave this place, so he had some knowledge of what he was going to do," he told CNN's John King. "(I) believe this was intentional -- it's two counts of murder and then suicide."
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 28-year-old Cox-Powell disappeared.
Before he set fire to his home, Powell sent his attorney an e-mail saying simply: "I'm sorry. Goodbye."
He also sent e-mails to his pastor and others just minutes before the fire, giving instructions on how to handle his end-of-life business, according to Troyer. Powell donated his sons' toys and books to a local charity, seemingly to clean house, he said.
The spokesman added that authorities found two five-gallon cans of gas in the home, one of which appeared to have been lit right next to the bodies, which were found together in the same room.
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.
The deaths of the Washington man and his two sons may mean the disappearance of the children's mother may never be solved.
Still, the case remains open and investigators vowed to pursue it until the point of closure.
"I promised the Coxes I wasn't giving up and I'm still not because we want to get some closure here," West Valley City, Utah Police Chief Buzz Nielsen told reporters Monday. A team of detectives was sent to Washington in the aftermath of the fatal fire.
"The case is still active; we're not closing the case. We still got things that have not been resolved," he said.
Powell, who was never charged in his wife's disappearance, was embroiled in a bitter custody dispute with her parents.
"Words can't describe any of this ... I can't imagine any of us going through what Chuck and Judy Cox have gone through, losing their daughter and losing these dear little boys," attorney Anne Bremner, who represented Charles and Judith Cox in the disappearance of their daughter, told HLN's Vinnie Politan.
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, Bremner said.
When asked Monday whether she feared Powell would ever hurt his children, Cox-Powell's sister, Denise Cox, responded: "Absolutely."
"From the get go, from when he was named a person of interest, the boys should have been taken away from him, and he should not have been allowed visitation," she told HLN's Nancy Grace.
The children had started "opening up and talking to my parents about what happened that night," in recent months, she said.
"The boys really, once they started opening up to our family, they started closing down on their dad," the sister told HLN.
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.
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.
The weekend's 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.
After 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 my 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, with debris landing on lawns blocks away.
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.
CNN's Chelsea Carter, Greg Botelho, Ashley Hayes, Sara Weisfeldt, Leslie Tripp and Dave Alsup contributed to this report.