Crews from the Forest Hills Fire Department responded with departments from Braddock, North Braddock, Braddock Hills, Edgewood, Swissvale and Rankin to multiple calls throughout Forest Hills and the neighboring area for Carbon Monoxide detectors going off. Calls began around 2:00 am and checked residences throughout the night. Crews found low levels of CO in the inside of the residences, higher levels on the outside. Gas company was notified to locate the source.