Elaine Redwine of Colorado Springs told ABC News she doesn't think her former husband, Mark Redwine, treats their 13-year-old son "very well."
"I would not put it past Mark to have done something to remove Dylan from the situation. You know, like 'if I can't have him, nobody will,'" Elaine Redwine said.
Dylan arrived at his father's home in Vallecito, Colo., Nov. 18, just one day before he disappeared.
Mark Redwine told police he left to run an errand and Dylan was gone when he returned hours later.
The FBI officially entered the search for Dylan Monday as divers and a boat equipped with sonar explored Vallecito Lake, The Denver Post reported.
A pair of search dogs in separate incidents had alerted handlers to scents they detected at the south end of the lake.
"Foul play is definitely something we are looking at, but we're hoping it's a runaway case and that Dylan will show up and will be fine," said sheriff's spokesman Dan Bender.