JOSH Powell's note was simple and short, a farewell to the world after two years of being scrutinised in the media, hammered by police and questioned by judges, prosecutors and social workers, living his life under a microscope since the day his wife vanished.

"I'm sorry, goodbye," Powell wrote in an email to his attorney just minutes before authorities say he set fire to his home, killing himself and his two young sons days after he was denied custody and ordered to undergo a psycho-sexual evaluation.

The Sunday blaze at Powell's home brought yet another twist in the very public scandal that began when Susan Powell vanished in 2009. The case had since spiralled into a salacious saga of finger-pointing and accusations of sex and lies - and now the unthinkable loss of two young lives caught in the crossfire.

A social worker brought the two boys to Josh Powell's home Sunday for what was to be a supervised visit. They rushed toward the home, leaving the social worker behind. By the time she got to the door, Powell had let his sons in but locked her out, Graham Fire and Rescue Chief Gary Franz told AP.

Pierce County sheriff's Sgt. Ed Troyer said emails Powell sent just prior to the blaze seemed to confirm that Powell planned the deadly fire. He didn't elaborate on the content of the emails.

Jeffrey Bassett, who represented Powell in the custody case, said he received an ominous email from his client just minutes before the fire.

"I'm sorry, goodbye," it read.

Investigators tried to fill in holes in the case yesterday with an arson investigation at the home and autopsies on Josh Powell and his sons, said Troyer. Local detectives also are meeting with police from West Valley City, Utah, who have been looking for Susan Powell.

Steve Richards, assistant chief of Graham Fire and Rescue, said crews were assessing the remnants of the home to determine how the fire began and what types of accelerants might have been used. He said responders arrived on scene about three minutes after getting the call and found flames already through the roof.

"It was just devastation," he said.

Fire investigators were slowly moving around the home this morning, measuring areas both inside and out. All the bodies were found Sunday in one room in the middle of the home, Mr Troyer said.

A candle light vigil was held overnight for the boys outside the 7-year-old's school in Puyallup.

The investigation will include a closer looks at Josh Powell's last movements and messages.

Chaplains have been working with the family of Susan Powell.

Josh Powell's father also was informed of the death in the jail in Tacoma where he's being held for investigation of voyeurism. He's on suicide watch, Mr Troyer said.

The Washington Department of Social and Health Services said the social worker who brought the boys to Josh Powell's home for what was to be a supervised visit is "suffering from grave emotional trauma as a result of the horrific event." The department will conduct a formal child fatality review.

She did all she could, Mr Troyer said.

The social worker called her supervisors to report that she could smell gas. Moments later, the home burst into flames, igniting an inferno that neighbours said rattled their houses.

Some sort of accelerant was used to make the house burn faster, Mr Troyer said. "It burned fast and hot."

Susan Powell, a pretty 28-year-old mother of two, was reported missing December 7, 2009, after she failed to show up for her stockbroker job in Utah.

Authorities in the couple's hometown of West Valley City, near Salt Lake City, quickly turned their attention to Josh Powell. He's been the only "person of interest" in the case, but had repeatedly denied any involvement in her disappearance.

"I would never even hurt her," a tearful, red-eyed Josh Powell told CBS' Early Show in August. "People who know me know that I could never hurt Susan."

About a month later, police spent 12 days in the remote central Utah desert looking for clues, and Josh Powell and his father, Steven, quickly disappeared from the limelight. The search area around Topaz Mountain, a popular spot for rock and gem hunters, was about 30 miles south of where Josh told police he went camping with his two children in the hours before his wife's disappearance - his steadfast alibi.

On Sunday, the lawyer for Susan Powell's parents, Chuck and Judy Cox, told AP the children had started talking to their grandparents about things they remembered from the night their mother vanished.

"They were beginning to verbalise more," said attorney Steve Downing, whose clients had custody of the children. "The oldest boy talked about that they went camping and that Mummy was in the trunk. Mum and Dad got out of the car and Mum disappeared."

Police turned up no clues in their desert search, but a day before ending it, Steven Powell, 61, was arrested at his Washington state home and accused of secretly videotaping his daughter-in-law, other women, and young girls taking baths and sitting on the toilet in neighborhood homes.

The elder Powell is now jailed and facing child porn and voyeurism charges. He claimed in previous television interviews that he and Susan Powell were falling in love and even implied a sexual relationship had occurred.

"Susan was very sexual with me," Steven Powell said in one interview at the time. "We interacted in a lot of sexual ways because Susan enjoys doing that."

Susan's father denied the allegations and said Steven Powell had been initiating unwanted sexual advances, and that his daughter had no interest in her father-in-law.

The children, 5-year-old Braden and 7-year-old Charles, were ordered by a judge to then go live with Susan's parents as the parallel cases were investigated.

The custody matter got so heated that at one point a court commissioner in Washington state ordered Chuck Cox and Josh Powell to keep 500 feet apart.

Custody hearings continued, with the latest on Wednesday, during which Josh Powell pleaded with a judge to return his children to him.

"For over four months already, my interactions with my sons and many other aspects of my character have been investigated and documented by" social services, he wrote in an affidavit to the court. "I have proven myself as a fit and loving father who provides a stable home even in the face of great adversity. ... It is time for my sons to come home."

But the judge ruled against him, ordering the children to remain with Susan Powell's parents, at least until Josh Powell underwent a psycho-sexual evaluation, a process more often used as an assessment tool by courts to determine whether a defendant, largely in sex crimes cases, is likely to reoffend. In this case, the judge ordered it in light of the explicit material found on computers inside Steven Powell's home that led to his arrest.

Sherry Hill, a spokeswoman for the Washington state Department of Social and Health Services, said the social worker who was with the children Sunday was not a Child Protective Services employee but a contract worker with a private agency that supervises visits for the state.

"The visit supervisor for this particular agency had taken the children to the home. When she does that, she sits through the visit and might take notes on her observations," Ms Hill said. "She pulled up in the car, and the kids ran out ahead of her. He closed the door and locked it. She wasn't able to get in, and that's when she smelled gas."

Mr Downing called it "the most horrifying thing you can imagine happening ...The Coxes are absolutely devastated. They were always very fearful of him doing something like this, and he did it."

Mr Bassett said he represented Powell free of charge because "every parent deserves the right to an attorney." Powell called or emailed him at least once a day, and often more than that, and in their conversations "he never once admitted doing anything regarding Susan. In fact, he denied it."

Sgt. Mike Powell of the West Valley City Police Department in Utah said it was too soon to say how Josh Powell's death may impact their probe.

"Quite frankly, this has obviously quickly unfolded up in Washington and we're obviously just working through the details ourselves here," said Sgt Powell, who is not related to the family.

Sgt Powell did not return a telephone message later in the evening Sunday regarding the children's assertion that their mother had been in the trunk of the family's car on the night she disappeared.

In a statement Sunday night from West Valley City police, authorities said the investigation will remain ongoing as they work to find Susan Powell. It said Utah police were headed to Washington state to work with investigators there in the continuing probe.

Kirk Graves, 39, of West Jordan, Utah, whose wife is Josh Powell's sister, said they were stunned by the news.

"We never contemplated the idea he would do something like this. You just don't expect it from a father," he said. "His world was falling apart around him and he was going to lose his boys and get arrested for Susan's disappearance. He's a narcissist and he has no love for anyone but himself."