DALLAS – Tests to detect carbon monoxide in Dallas´ compressed natural gas (CNG) patrol cars have been unable to provide answers as to how an officer got sick after driving a retrofitted vehicle, reported The Dallas Morning News newspaper. Carbon monoxide was not detected in the cabs, fuel lines, or exhaust systems of any of the 70 CNG cars, according to city officials. An officer said she could smell natural gas while driving one of the converted vehicles and became sick. She was hospitalized and found to have elevated levels of carbon monoxide. Once the vehicles have been deemed safe, they will no longer be used to answer calls, city and police leaders told the Morning News. Officers who drive less, such as supervisors and community officers, will use the vehicles instead. In a show of support for alternative-fuel vehicles, Dallas Police Chief David Kunkle said he will trade in his gasoline-powered vehicle for a CNG model.