SEATTLE (Reuters) - The two boys of a man who blew up his Washington state home, killing himself and his sons, suffered chop wounds in their final moments and died of smoke inhalation, a law enforcement official said on Monday.

Josh Powell was in a bitter custody battle with his missing wife's parents when he locked his children, 5-year-old Braden and 7-year-old Charles, in his house in Puyallap, Washington, and ignited 10 gallons of gasoline and accelerants on Sunday, police say.

The immediate cause of death for both Charles and Braden was carbon monoxide poisoning, or smoke inhalation, but the "other significant condition" in the two boys' deaths was chop wounds, said Melissa Baker, an investigator with the Pierce County Medical Examiner's Office.

Charles suffered a chop injury to his neck and Braden was struck in the head and neck, Baker said. She could not reveal what weapon might have been used to attack the boys.

The explosion rocked Powell's rental home shortly after a Child Protective Services worker dropped off the two boys for what was supposed to be a supervised visit.

The youngster's cause of death is just the latest detail to emerge in the grim saga since Sunday.

Family lawyers said the boys had recently begun to speak about their mother's 2009 disappearance, in which the father was a suspect.

In the months leading up to the fiery blast, one of the boys drew a picture for his first-grade teacher at the beginning of the school year that Steve Downing, a lawyer for the parents of Powell's wife, said could shed light on the circumstances of her disappearance.

"His father was driving the van and he and his brother were in the back seat. His mother was in the trunk," Downing said of the drawing.

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

The son's recalling of the incident contradicts the story his father told police at the time of his wife's disappearance more than two years ago.

At the time of Susan Powell's disappearance, Powell told police that he took his two sons to go camping in subfreezing temperatures just after midnight on December 7, leaving his wife at home, according to court documents.

His wife had vanished when he returned home that afternoon, he told police. Court papers show that authorities found no signs of a campsite where Powell said he took his children, then aged four and two.

Josh Powell, 36, had sent e-mails to his church pastor, a cousin, and others shortly before starting the fire, Pierce County Sheriff's Sergeant Ed Troyer told a news conference.

"They do show that he was intent on doing this," Troyer said of the e-mails. "He had taken boxes of toys and donated to Goodwill sometime over the weekend."

Puyallap, where the Powell family lived most recently, is 30 miles south of Seattle.

'GRAVE EMOTIONAL TRAUMA'

Medical examiners found Powell died of carbon monoxide poisoning, Baker said. They ruled his death a suicide.

Powell had prevented the case worker from entering the house and used two 5-gallon cans of gas and accelerants scattered throughout the house to start a fire and ensure that it spread rapidly, Troyer said.

The uninjured case worker, who reported smelling gas before the blast, was "suffering from grave emotional trauma" after the event, a statement from the child protective agency said.

Powell's children had been living with their father until September, when police arrested his father Steven, with whom he lived, on charges of voyeurism and pornography after police said they discovered thousands of photos and videos of women and girls, taken without their knowledge, at his home.

His daughter-in-law, Susan Powell, was among those photographed, police say.

After the arrest, custody of the boys was granted to Susan's parents, Charles and Judy Cox, until Josh Powell agreed to undergo a psycho-sexual evaluation.

The boys "were beginning to remember things," once in the custody of their maternal grandparents, Downing said.

A judge denied custody to Josh Powell last Wednesday due to the "extreme child porn" found by police in the Powell family residence, Troyer said.

Steven Powell faces trial next month on the criminal charges related to the photos of unclothed girls taken while they were bathing, using the toilet and dressing.

"I have always been a loving and devoted father," Josh Powell said in court documents in December. "My sons and I have a very close relationship and I am working diligently to get them back in spite of the lies people are saying about me."

Investigators from the Salt Lake City suburb where Susan Powell lived with her family before disappearing had traveled to the site of the blast to continue what remained an active investigation into Susan Powell's disappearance, a police spokesman said.

"We had hoped many times for the arrest of Josh Powell," Anne Bremner, a lawyer representing Susan's parents, told Reuters, calling the boys' deaths "beyond horrific."