CASTLE ROCK – The biological mother of a child killed in an Arapahoe County apartment Wednesday says she believes her son may have been trying to stick up for a victim of sexual assault.

The boy’s father, Brandon Johnson, 26, was arrested Friday and charged with his murder. He is accused of killing Riley Johnson, 6, and sexually assaulting his live-in girlfriend at knife point.

“I believe my son is a hero,” Rachel Johnson told 9NEWS Friday.

Rachel and Brandon are now divorced. Brandon got custody of Riley and the couple’s other son, after Rachel was convicted of misdemeanor child abuse involving a 2012 case.

She moved to Illinois she said to get her life back together, take classes on parenting, and try to get her children back.

Last year, Rachel said Brandon asked her to take back the couple’s older son, but kept Riley. She was trying to fight for custody of Riley.

“My caseworker called me and told me she was going to close my case so I could come back out here and fight for Riley,” Johnson said. “And then I get a phone call saying my son is gone.”

Rachel said her ex-husband was never violent, but said he was controlling. She detailed an instance when Brandon tried to force himself on her in front of Riley.

“My son said daddy get off of mommy and he just let me go,” she said.

She believes a similar situation may have played out in an Arapahoe County apartment Wednesday.

Rachel describes Riley as a funny kid, always quick with a joke. She said he was full of life.

When she went to identify her son’s body at the coroner’s office, Rachel said investigators didn’t want her to see him like that, so they instead gave him a lock of her son’s hair.

“I just want to hold my son and I think that’s why they wouldn’t let me see him… because I’d probably end up trying to hold him close to me because that’s all I want to do right now is hold my baby and tell him how sorry I was.”

(© 2016 KUSA)