GRAHAM, Wash. - Josh Powell's boys were coming for a visit, and he had preparations to make.

He boxed up their books and toys and took them to a charity. He carried heavy cans of gasoline inside his house and set aside a hatchet.

He thought about what to write in the final emails he'd send out: where people could find his financial accounts, how they could shut off his utilities. He didn't mention his wife, Susan Powell, missing for the past two years.

When the boys finally arrived in a car driven by a social worker Sunday, everything was ready to go. They ran inside to see him. He locked the door before the social worker could reach it.

She could smell the gas, which by now was splashed throughout the home. She called for help. The flames rose.

As authorities continued searching through the charred rubble of the home Monday, they released new details about what they described as Powell's deliberation in killing himself and his children, who had been removed from his care - a horrifying climax to a long, bizarre saga. They found the two 5-gallon gas cans and a hatchet inside.

"This was definitely a deliberate, planned-out event," said Pierce County Sheriff's Detective Ed Troyer.

Smoke inhalation was the primary cause of death for Josh Powell and his two young sons, Pierce County Medical Examiner's Office investigator Melissa Baker said Monday evening.

But the boys 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.

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 them (the bodies)."

The Utah police chief heading the investigation into Susan Powell's disappearance, Buzz Nielsen of the West Valley City Police Department, flew to Washington state Monday and vowed that the case will remain open.

Detectives hope to interview the father of Josh Powell, Steve Powell, who remains in jail on child pornography and voyeurism charges, Nielsen said. He identified Steve Powell as a "person of interest" in the disappearance but then said no arrest was imminent: "He's not in our sights."

Josh Powell had long been identified as a person of interest in his wife's disappearance, and last week a judge denied another request for his boys to be returned to him.

Powell claimed that the night his wife vanished in December 2009, he took the boys on a midnight camping trip in freezing temperatures - a story neither her parents nor police believe.

Asked why Josh Powell had never been arrested, Nielsen said: "In a criminal case of this nature, you've got one shot. You've got to make sure everything is done right." He said investigators were making progress and had hoped to make an arrest within the year, but declined to discuss what evidence they might have had against Josh Powell.

If Powell did know what happened to his wife, he appears to have kept that secret when he died.

Troyer said that 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 such as 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."

Police who arrived at the Utah home to look for the family found two fans pointed at a damp spot on the floor, but no trace of Susan. Her body has never been found despite intensive searches in Utah and Nevada.

Less than a month after the disappearance, Powell moved the boys to his father's home in Puyallup, south of Seattle. He maintained custody of the boys as the scrutiny upon him intensified. Steve Powell claimed on national television last year to have had a flirtatious or even sexual relationship with Susan - something her family adamantly denied.

Last fall, when the elder Powell was arrested in a voyeurism and child pornography case, the state turned the boys, Charlie, 7, and Braden, 5, over to Susan Powell's parents, Charles and Judy Cox.

The tragedy left the Coxes devastated. They opened their homes to reporters Monday to give a glimpse of the lives the boys led there.

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 the visit - 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 she 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.

Associated Press writers Brian Skoloff in Salt Lake City and Martin Griffith in Reno, Nev., contributed to this report. Johnson reported from Seattle.