The Washington man who authorities believe killed his two sons and himself by setting fire to his house first gave toys and books to charity, sent multiple goodbye e-mails and doused the home with gasoline, they said.

Such evidence suggests that Josh Powell planned the murder-suicide for some time, according to Ed Troyer, spokesman for the Pierce County, Washington, Sheriff's Department.

"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."

Powell, a suspect in the 2009 disappearance of his wife, 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.

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 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.

Late Monday, the Pierce County Medical Examiner's office released the cause and manner of death on Powell and his sons. All thee died of carbon monoxide poisoning. Both boys also suffered chop injuries to the neck; the younger boy had the same injury to his head. Powell's death was ruled a suicide, while his sons' deaths were determined to be homicides.

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 opens 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.