Story highlights In an affidavit filed last week, Powell said, "I am standing tall for my sons"

Josh Powell killed himself and his sons in a "double homicide," an official says

"This was all on him. He set this up, he did it," the sheriff's spokesman adds

Powell has said he last saw his wife, Susan Cox-Powell, in December 2009

Minutes after locking out a social worker who'd brought his two sons for a visit, a man in Washington state blew up his house on Sunday -- killing himself and his boys, whose mother went missing more than two years ago, a sheriff's spokesman said.

Pierce County Sheriff's Department spokesman Ed Troyer told CNN that the explosion "was done intentionally," saying Josh Powell committed a "double homicide" in addition to killing himself.

"This was all on him," Troyer said of Powell and the blast. "He set this up, he did it."

Days earlier, in an affidavit seeking custody of the children, Powell said that he had proven himself as a "fit and loving father who provides a stable home even in the face of great adversity."

"My inherent resilience as a person makes it increasingly difficult for (critics) to pursue their agendas," he wrote in the document, which was filed Wednesday in a Pierce County court, referring to those blaming him for his wife's disappearance. "I am standing tall for my sons."

On Sunday, authorities said Powell met a foster-care worker who had brought his two boys -- ages 5 and 7, having both celebrated birthdays in January -- to their father's residence for what was supposed to be a supervised visit.

As the children got close to the door, Powell brought them inside but prevented the case worker from getting in as well, according to Troyer. She tried "pounding the doors, trying to get in," with no success, he added.

The case worker reported smelling something similar to gas, said Graham, Washington, Fire and Rescue Deputy Chief Gary Franz. About two minutes later, as she was calling her supervisor, the house exploded.

"All the walls in the house were on fire, almost immediately," neighbor Ryan Mickle told CNN affiliate KIRO , adding that the blast "shook my whole house." "I didn't hear anybody inside. It was quite a scene."

Authorities first heard of the explosion in Puyallup, Washington, around 12:15 p.m. (3:15 p.m. ET), according to Troyer.

The fire was still "burning hot" and smoke continued to rise from it some four hours later. By 7 p.m., and despite initial concerns that the house collapse, the bodies finally had been recovered, said Troyer.

While a medical examiner had yet to definitively identify the bodies, the sheriff's spokesman said Sunday night that "we believe it is the three of them" -- referring to Josh Powell and his two sons.

"This was something that was done deliberately and intentionally, and the plan was carried out very quickly," he added of authorities' initial take of what happened, noting that "obviously" an "accelerant" was used to fuel the blast.

There were no other injuries and no other houses in the neighborhood in Puyallup, which is about 10 miles southeast of Tacoma, appeared to be damaged, said Troyer. He added a chaplain was at the scene with the social worker, who he described as physically OK but emotionally devastated by the blast.

Troyer said the sheriff's department has copies of a brief e-mail that had gone out to Powell's family and friends, "stating that he couldn't live with what was going on."

Authorities haven't confirmed the message came from Powell, but "we have no reason to believe it was not from him," he said.

"It shows that there was intent, and this was what he intended to do -- which is to kill his children and kill himself," Troyer said of the e-mail.

In the affidavit seeking custody of his children from his wife's parents that was filed last week, Powell claimed that his sons "sustained severe injuries in the care of their grandparents," adding that, "for the sake of my sons, we can all do better."

"If anyone wants to be honest with themselves and the situation, the real story is not that anyone is a bad person, including me," he said. "The story is one of overcoming and rising above what many people think are insurmountable challenges."

The Washington court denied the motion from Powell to gain custody, said Jeffrey Bassett, a Washington lawyer who was representing Powell. Troyer said a judge also ordered Powell to "go through different types of evaluations and counseling," though supervised visits could continue.

Powell had said that he last saw his wife, Susan Cox-Powell, on a cold December night in 2009.

That night, he said that he and his two sons -- then ages 2 and 4 -- left after midnight to go camping in below-freezing weather in a desert area in Tooele County, Utah. Powell-Cox's sister eventually reported the mother, who would now be 29 years old, as missing.

A month later, Powell and his children moved from Utah to the state of Washington.

While there have been no arrests or charges filed related to his wife's appearance, Josh Powell was identified as the lone "person of interest," according to Sgt. Mike Powell -- no relation to Josh or Susan Cox-Powell -- with the West Valley City, Utah, police.

The woman has not been found.

Through their attorney Anne Bremner, Cox-Powell family issued a statement Sunday asking "for time, privacy and prayers after today's horrific events."

Since September 2011, Josh Powell had been engaged in ongoing custody battle with his wife's parents Judith and Charles Cox, according to Washington state court records.

As part of the investigation after Cox-Powell's disappearance, authorities last year searched a house in Washington state where Powell, his two sons and his father, Steven Powell, were living. In the course of the search, detectives from West Valley City "discovered numerous images and recordings of adult and juvenile females," according to a release from the Pierce County sheriff's department.

Steven Powell subsequently was arrested, in September, and charged with 14 counts of voyeurism and one count of possessing images of children engaged in sexually explicit conduct, according to court documents.

He was still in custody on those charges in Pierce County as of Sunday, according to official records posted online.

The house that blew up was on a cul-de-sac, about four blocks from the one that had been searched in 2011, Troyer said.

Authorities involved with the case were struggling Sunday to come to grips with what had happened, from the violence of the explosion and the two boys being caught up in it.

Troyer said many law enforcement agents in Washington state have worked with their counterparts in Utah on the Susan Cox-Powell case, as well as spent time playing, coloring and getting to know her sons.

"Our guys have spent a lot of time and effort and blood, sweat and tears on this case," said Troyer. "We have detectives who are close with the kids, ... and know them. The whole thing is just sad."