The 4-year-old's father, an Army sergeant, returned home deployment in Afghanistan on Oct. 3 and visited the children at their mother's Yelm home three days later.
Police say the father noticed right away the state of decay of the 4 year-old's teeth and a significant apparent cigarette burn underneath the left forearm.
"He got burned by a cigarette or in the boy's words, he was set on fire by the cigarette," said Chief Todd Stancil with Yelm Police.
The father took the boy home and called police. Investigators say the 4-year-old told them his 18-year-old half brother Timothy Rowland did it.
The boys' mother denies the claims.
"(Timothy's) been very good to him," said Danielle Hathaway. "I don't believe that he would have done that. He loves him."
But the boy told police they were mean to him.
"He was afraid of them and they liked to hurt him," Stancil said. "And mentioned that he had been kicked in the stomach. "
Hathaway says the boy was not afraid of them.
"He's not afraid... I know he's not," she said.
She and the boy's father are divorced, and she believes this is all a vendetta.
"I have an ex-husband that has been after me since our divorce in regards to trying to take the children away from me," Hathaway said.
She says there are no cigarette burns and a burn from an iron on the boy's arm was from an accident with her. And the boy has dental hygiene problems with his teeth that she can't afford to get fixed.
"This is nothing but lies," she said.
KOMO News tried to interview the boy's father, but his military leaders at Joint Base Lewis-McChord said he wasn't available.
Meanwhile, police say Hathaway is still under investigation, but Rowland was placed under arrest and charged with child assault. He's been booked into the Thurston County Jail and ordered held on $200,000 bail. The boy is staying with his father.