Josh Powell with sons Charlie and Braden
Amber Hardman/Hardman Photography/Polaris
02/06/2012 at 06:40 PM EST
One of Susan Powell's sons drew a picture of her in the trunk of the family car with father Josh Powell at the steering wheel, according to Susan's father and a family attorney.
In interviews with PEOPLE, Powell's dad Chuck Cox said he believed it was younger son Braden who drew the picture, while lawyer Steve Downing said it was older son Charlie.
Both boys and Josh were killed
Sunday when Josh blew up his house. Josh, who had long been a person of interest in Susan's disappearance
, had claimed that she went missing while he and the boys went on a camping trip without her in December 2009.
Cox – who had fought Josh Powell for custody of Braden, 5, and Charlie, 7 – and Downing say the boys had been opening up about the time around their mother's disappearance.
At a preschool day camp about two years ago, Cox tells PEOPLE, Braden and Charlie were drawing pictures when Braden came up with a picture of a minivan with three people in it.
"That's us going camping," Braden said, according to Cox.
Braden identified the three as he, his brother and his father. When asked where their mother was, according to Cox, he said, "She's in the trunk."
Cox also quoted Braden as saying, "We went camping and mommy and daddy got out and mommy never came back."
Cox was not a witness to this. He did not say how he got the information about the drawing or the boy's comments.
In a separate interview with PEOPLE, Downing says it was Charlie, not Braden, who had drawn the picture and spoken of his mother not coming back from the camping trip. Charlie was 4 years old at the time of the disappearance while his brother was only 2.
Downing says he got his information about the picture from police and other sources, and not from Cox.