According to investigative records obtained by the Grand Junction Daily Sentinel, 24-year-old Heather Jensen told police she drove up to the Powderhorn area to meet a "male friend." The two were inside the man's truck for 90 minutes while the boys stayed in the running SUV parked nearby.
She then called 911 to report her 2-year-old boy William was unconscious, not breathing and he had no pulse shortly after she left the man and his truck.
Jensen told dispatchers around 7 p.m. Tuesday that her 4-year-old boy Tyler was barely breathing.
William was pronounced dead at the scene, while Tyler was flown to St. Mary's Hospital before being transferred to Children's Hospital in Denver.
The children's step-grandfather told 9NEWS' sister station KKCO that Tyler is unlikely to survive due to organ damage. Robert Mathena was unclear whether the children suffered from carbon monoxide poisoning. Mathena also said that Colorado Department of Human Services assumed emergency state custody of Tyler after the incident.
The boys' father, Eric Jensen, died in a car accident in Rifle last month.
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.
If you have a news tip for 9 Wants to Know Investigator Melissa Blasius, email her at melissa.blasius@9News.com.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)