Two people were taken to the hospital after an elevated carbon monoxide level was detected inside a home in Avon on Wednesday afternoon.
The Avon fire chief said it happened on Saddle Crossing, where homes in the complex use natural gas for heat.
When fire crews arrived, they said the carbon monoxide meter was elevated.
The patients complained of symptoms that coincide with carbon monoxide poisoning.
Fire officials also said a nearby home also had high CO levels.
"We stared medical care and we started an investigation to see what was propagating the carbon monoxide," said Captain Aaron Gelber, Avon Fire Department.
Connecticut Natural Gas is investigating to determine a cause.
Carbon monoxide is colorless and odorless so fire officials remind folks that a CO detector is important to have in each home.