(CBS News) A judge in Tacoma, Wash. dismissed a child pornography charge against Steven Powell, the father of Josh Powell on Monday.
Earlier this year, Powell killed himself and his two sons, leaving many unanswered questions about his missing wife, Susan.
Her parents say they believe her father-in-law can explain what happened to her and her family.
"I still have things of the kids," Judy Cox told "CBS This Morning." "And it's hard if you run across them. It makes me cry."
For Chuck and Judy Cox, memories are all they have left of their two grandsons.
"Their dad made a lot of promises and he never kept them," Judy said.
In February, during a supervised visit with their father, seven-year old Charlie and five-year old Braden were killed in an apparent murder-suicide. Police say Josh Powell locked a social worker out of his home and attacked his two sons with a hatchet before starting a gas-fueled blaze that consumed the house within minutes, killing everyone inside.
In a 911 call, social worker Elizabeth Griffin-Hall was heard exclaiming, "Yes, he exploded the house! He exploded the house. He blew up the house and the kids!"
"I already felt numb, but it took my breath away," Judy said.
"The fact that he killed those boys," said Chuck, "and the way he killed them, what kind of monster it -- it pointed out what kind of a person he really was."
Josh Powell's father, Steven Powell, is now on trial, charged with fourteen counts of voyeurism. During a search of the Washington state home the elder Powell shared with his son, police found secretly recorded images of young women, including Josh's wife, Susan Cox Powell, who's been missing since 2009.
With the wounds of their grandsons' deaths still fresh, the Coxes hope Steven Powell will finally provide them with the answers they've been looking for.
"He needs to tell the truth and tell us what happened to our daughter," Chuck said.
From the beginning, Josh Powell maintained Susan disappeared from their Utah home while he and his sons were on a midnight camping trip in the Utah desert.
Chuck and Judy Cox say their former son-in-law was very close with his father, and they're convinced Steven Powell knows what happened to their daughter.
"We're hoping that, when he realizes that he's not gonna get out on some technicality, that this is real ... that he would be willing to go ahead and admit his part in it and tell us where she is," Chuck said.
Almost three years after Susan's disappearance, but only three months after their grandson's brutal deaths, the Coxes hope the trial might bring them some sort of closure. And maybe even some sort of justice for their daughter.
"We still want to know the truth," Judy Cox said. "No matter what it is, we feel we need to know."
To see Bill Whitaker's report, click on the video in the player above.