By Jessica Mintz

TACOMA, Wash (Reuters) - Over a thousand mourners flocked to a memorial service in Washington state on Saturday for two young boys killed when their father burned their house to the ground with himself and his sons inside.

Sunday's murder-suicide, which claimed the lives of Josh Powell and his sons, Charles, 7, and Braden, 5, capped a grim family saga that began over two years ago with the disappearance of the boys' mother under suspicious circumstances in Utah.

The boys, who were at the center of a bitter custody fight between Powell and his wife's parents, died after their father set off a fiery explosion that consumed his house as a social worker, locked outside, watched in horror.

Medical examiners later said the two brothers, killed during what was to have been a supervised visit with Powell, had suffered chop wounds from hatchet blows inflicted by their father moments before all three died of smoke inhalation.

At the memorial service, the maternal grandparents of the boys, Chuck and Judy Cox, sat in the front row of pews as a children's choir sang "Amazing Grace" and the memorial service began, with images of the boys smiling and playing with family shown on a large screen.

"Throughout this trial we have been enduring for the past two and a half years now, we felt the support of so many people around the world," a visibly emotional and choked up Chuck Cox told mourners, holding hands with his wife Judy.

"It helps us to be strong. It helps us to know that there are good people in the world, good people that fight against evil," he said.

A single closed, light blue casket rested at the front of the church, with red roses and orange and yellow Gerbera daisies in a large arrangement resting on top. Nearby, framed photos of the boys were displayed next to a vase of white lilies.

Over 1,000 friends, family and members of the community filled the church in Tacoma as Pastor Tim Atkins applauded both sides of the family for putting aside what had happened to all be present.

Josh Powell's family sat in the upper level of the two-story church, while the family of his wife, Susan Powell, sat front and center in the first-floor pews. The two groups did not appear to interact.

Pallbearers including Chuck Cox carried the casket into a white hearse at the conclusion of the hour-long service as the sun emerged from the cloud cover and light rain of what had begun as a grey, grim Pacific Northwest winter day.

During the service, teachers and caretakers of the boys described them both as lovers of nature, with keen intellects and compassionate hearts. Both were said to have a particular knack for losing their socks.

Mourners left teddy bears, balloons, and flowers for the family alongside framed portraits of the boys with their mother Susan in the lobby of the church.

The boys will be laid to rest in a Puyallup cemetery, about 20 miles southeast of Tacoma, in a private burial later on Saturday.

CUSTODY STRUGGLE

Last week, a judge denied Josh Powell's bid to regain custody of his sons, citing the "extreme child porn" found last September in the house he rented near Tacoma.

That discovery had led to the arrest of the boys' grandfather, Steven Powell, who had been living with them and whose vast pornography collection included images of their missing mother. He now awaits trial on criminal charges related to the photos.

The boys had been ordered to remain in the custody of their maternal grandparents at least until Josh Powell, 36, submitted to a psycho-sexual evaluation. Four days after that decision, the boys were dead.

Pierce County prosecutor Mark Lindquist said earlier this week that he considered "the murder of those children a confession to the murder of Susan Powell" and that he viewed the ongoing investigation into her fate as a murder investigation.

Family lawyers have said the boys had recently begun to speak about their mother's disappearance, which took place in December 2009 when the brothers were ages 4 and 2 and the family was living near Salt Lake City.

In recent months, one of the boys drew a picture for his first-grade teacher depicting his father driving a van with the brothers in the back seat and their mother in the trunk, according to Steve Downing, an attorney for Susan Powell's parents.

When the teacher asked the boy about the drawings, "Charlie told her, 'Then my mom and dad got out of the car and my mom got lost,'" Downing told Reuters.

At the time his wife was reported missing, Josh Powell told police he had taken his sons camping in subfreezing temperatures just after midnight on December 7, leaving his wife at home, and that she had vanished by the time they returned that afternoon, according to court documents.

Investigators now presume she was slain by her husband, though her body has yet to be recovered.

The grandfather, Steven Powell, 62, was charged in September with 14 counts of voyeurism and one second-degree charge of possession of child pornography. Shortly after that, the boys were placed in the custody of their maternal grandparents.

"These were two happy little boys caught in the middle of a family dispute, caught in the middle of evil that some other people were doing," said Dean Curry, lead pastor of the Life Center church in Tacoma, an Assemblies of God congregation.

"But they were innocent and they deserve a moment ... so we're going to give it to them," he told Reuters on Friday.

(Writing by Mary Slosson and Steve Gorman; Editing by David Bailey and Cynthia Johnston)