MESA COUNTY, Colo. - After the death of a second young boy who'd been left in an SUV, deputies say the case remains under investigation.

On Nov. 27, the boys' mother, Heather Jensen, called 911 and said Tyler, 4, was struggling to breathe, and his 2-year-old brother William was not breathing.

Tyler was taken to Children's Hospital in Aurora. The Mesa County Sheriff's Office confirmed Tuesday that the child had died.

Heather Jensen admitted leaving the boys alone in her Toyota 4-Runner, with the engine running, for approximately 90 minutes while she met a male friend, according to a search warrant affidavit obtained by the Grand Junction Daily Sentinel.

The public information officer for the Mesa County Sheriff's Office, Heather Benjamin, said that no arrest documents have been obtained, filed or executed.  Benjamin confirmed to 7NEWS that investigators have not ruled out homicide or child neglect.

“I guess you could say nothing has been ruled out. This case is in the very early stages and continues to remain very active," said Benjamin.

William was pronounced dead at the scene. The cause of his death has not been released.

A doctor who treated Tyler told investigators Wednesday she did not believe the toddler suffered carbon monoxide poisoning, according to court records obtained by The Daily Sentinel.  The Sentinel obtained the records when they were public but they are now sealed.