Investigators with the Mesa County Sheriff's Office and the coroner's office were at the scene Tuesday night trying to determine what happened.
NBC affiliate KKCO reports 2-year-old William Jensen died Tuesday inside a red Toyota 4 Runner. His 4-year-old brother, Tyler Jensen, is on life-support at Children's Hospital in Denver.
The children's step-grandfather told KKCO he believes the two suffered carbon monoxide poisoning after they were left in the running vehicle by their mother, Heather.
The Mesa County Sheriff's Office says the coroner has completed the autopsy, but they are not releasing the results because it's tied in with the investigation.
"Once the cause and manner is released, a lot more information would make sense and pieces would fall together for the public. We are not at that point though," Heather Benjamin, a spokesperson for the Mesa County Sheriff's Office, said.
The boys' father died in a car accident in Rifle last month, according to KKCO.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)