(CNN) -- Authorities conducting autopsies on a family of four found dead in in their home in eastern Missouri said preliminary autopsy results showed that the family may have died from carbon monoxide inhalation, police said Friday.
The four -- a couple in their late 20s, their 4-year-old son and their 3-year-old daughter -- were found dead Thursday afternoon, said police in the city of St. Clair.
Police had been summoned to the house after the man's co-workers became concerned that he had not gone to work for several days, said police Chief Bill Hammack.
Inside, officers found the four bodies but no indication of foul play, he said.
Family members told police they had last been in contact with the family Tuesday, when he complained that he was feeling sick and nauseated, Hammack said.
According the Center for Disease Control, each year, nearly 500 people die in the United States from carbon monoxide poisoning, and as many as 20,000 visit emergency rooms for exposure from poorly maintained heating systems and gas-powered generators.
"Dangers from poorly maintained heating systems are really going to be the number one cause of carbon monoxide poisonings in the United States," Dr. Paul Garbe said on a CDC online video.
"CDC recommends that all homes have carbon monoxide detectors," Garbe said.
"I think it's a great investment. It's particularly important that you have carbon monoxide detectors near where people sleep at night. The worst location for a carbon monoxide detector is in the box without a battery," he said.