AURORA, Colo. -- Aurora police are investigating what caused the mysterious deaths of two young siblings who died in their sleep Thursday evening.

The 5-year-old boy, Robert, and his 4-year-old sister, Rhapsody, were found dead by a young cousin who could not wake them up from an afternoon nap, police said.

Aurora police officers arrived at their home, located on East Bethany Place and South Laredo Street, after receiving a 911 call from their mother, Faye Henderson, at about 5:30 p.m.

The children's 29-year-old father, Robert Henderson, told investigators that Robert and Rhapsody were not feeling well that morning. He said that the boy and girl appeared to feel better after they ate lunch and went to their bedroom for a nap.

When the children's mother, Faye Henderson, 28, arrived home from work, she did not bother them because she was told that they were still sleeping.

Later in the evening, while Faye Henderson was talking with her sister, the children's young cousin went into their room to try to wake up the boy and girl. When she couldn't stir them, the girl ran to the two women to tell them that something was wrong.

Crews from Xcel and the Aurora Fire Department have ruled out carbon monoxide poisoning and natural gas as the causes of their deaths.

No one else in the house has reported feeling ill.

The children's father, mother and aunt were taken to the Aurora Police Department Thursday evening for questioning.

No charges have been filed, but police are treating the case as a suspicious death.

Detectives say the children's parents are cooperating with the investigation.

Autopsy Results Inconclusive

An autopsy conducted on Friday gives little clues about how the children died.

It all now hinges on the toxicology results which may not be released for weeks, 7NEWS reported.

The coroner told 7NEWS that the children were normal and healthy, that there were no signs of struggle, no signs that they were smothered, and no signs of trauma.

The coroner also said that there were no obvious signs that the children got into anything poisonous, because there was no corrosion found in their esophagus. However, he said that there are some poisons like antifreeze that are not apparent in the system during a preliminary autopsy.

Detectives have not ruled anything in or out and are searching the house for any indication that the children "got into something."

A makeshift memorial filled with stuffed animals and toys has already started piling up outside of their home.