Josh Powell's brother-in-law said Monday that the motivation behind Powell's killing himself and his two sons
on Sunday afternoon may have been to make sure the boys kept quiet about their mother Susan's 2009 disappearance
"I think this was a last-ditch effort on his part to hurt everyone around him and escape all of the problems," Kirk Graves told PEOPLE. "Regardless of the impact on the boys, he just wanted to hurt everybody else."
Charlie, 7, and Braden, 5,
had become more forthcoming in recent months, according to family members and authorities. Charlie had even gone as far as to say his mother was "in the trunk" when his father took the boys camping on the night she went missing, according to Graves.
"They were becoming more fun, more vocal," Graves, who is married to Powell's sister, Jennifer, said Monday morning on NBC's Today show
. "They were acting like little boys again, which we hadn't seen in a while. … I imagine that part of Josh's motivation was, what could he do to hide that story?"
Ed Troyer, a spokesperson for the Pierce County Sheriff's Department, told Today
that Powell was "lying in wait" when a social worker dropped Charlie and Braden off at his home for a visit Sunday afternoon. Powell pulled the boys inside, shoved the worker out the door and blew up the home as the concerned worker called her supervisor, killing himself and his sons.
An investigation indicated that accelerants were used, Troyer said on Today
There was "absolutely nothing anybody could have done" to stop the double-murder suicide, Graves insisted. But that doesn't alleviate the stinging sense of loss that the family is now feeling all over again.
"Not only this a horrible, horrible thing, but it just came out of nowhere," Graves told PEOPLE. "It leaves you in a total state of shock. Why did he do this to the boys? I've been asking myself that I don't know how many times."
Powell, who denied wrongdoing
in his wife's disappearance, had been fighting to regain custody
of their boys, who were being cared for by their maternal grandparents. A judge had recently ordered Powell, whose father is the subject of a child pornography case
, to undergo a psychosexual evaluation, according to reports.
He sent an email to his lawyer moments before setting off the explosion that read simply, "I'm sorry, goodbye," authorities say.