(KRON) — Two people found dead in Santa Rosa on Tuesday are believed to have been killed by carbon monoxide poisoning, the City of Santa Rosa announced Thursday.

Santa Rosa’s police and fire departments went to 2705 Range Ave. just after 9:30 a.m. on Tuesday after the bodies were discovered. The city said a maintenance representative at an apartment complex went inside a unit for routine maintenance and found the two elderly adults unresponsive.

The Santa Rosa Fire Department went inside and determined that the subjects may have been exposed to carbon monoxide. An air monitor confirmed high levels of carbon monoxide were present.

A dog was found awake and alert in the bedroom, and it was turned over to Sonoma County Animal Control. All other units at the complex were ruled to be clear of carbon monoxide and no other residents had medical complaints, the city said.

A preliminary investigation suggests that the carbon monoxide poisoning came from a gas-fueled appliance, the city said. The county coroner’s office is still conducting autopsies and an investigation is ongoing.