AURORA, Colo. -- An overdose of a generic cough and cold syrup killed two Aurora children who died in their sleep in early August, 7NEWS reported.

Five-year-old "Killian" and 4-year-old Rhapsody Henderson apparently took a large dose of a store-brand liquid cold medicine, similar to Nyquil, before they took their afternoon nap on Aug. 8.

They never woke up.

Police have interviewed the children's parents, Robert and Fay Henderson. It is unknown if they will face any charges, but police have classified the deaths as "suspicious."

Meanwhile, the Colorado Bureau of Investigation is completing laboratory tests on the crime scene. Results of those tests should be available later this month.

Nyquil and equivalent store brands use Dextromethorphan, which affects the brain and is found in many over-the-counter cough syrups. It can be deadly in large quantities, but doctors say it likely would take several times the recommended dose to kill an otherwise healthy child.

The children's autopsy report, along with all the information and evidence gathered during the investigation, have been sealed by a court order. That's the kind of step police and prosecutors ask for when they anticipate filing criminal charges, 7NEWS reported.

Investigators have long suspected toxic ingestion of some type of over-the-counter drug as the cause of the childrens' deaths, 7NEWS reporter Julie Hayden reported. But there was no evidence that the children had tampered with or gotten into any dangerous substances on their own, she said.

The children's father had said that the two were not feeling well that morning. At lunchtime, the pair ate peanut butter and jelly sandwiches, watched a little television, then went back to bed.

When their father went to work that afternoon, their mother came home and was told not to bother them because they were sleeping. Later in the evening, when relatives dropped by and wondered why the children were still asleep, a young cousin found them dead in their beds.