SIERRA VISTA, Ariz. Health officials still don’t know the cause of a salmonella outbreak that has sickened at least 56 Sierra Vista residents since September. Officials with the federal Centers for Disease Control and Prevention have been in the area since January trying to figure out where the victims came down with the infection. They’ve interviewed people who came down with the food-borne illness and their friends and family. But they say they’ve had no luck and are working on a computer analysis to try to find a link. They’re also doing a phone survey to try to find a common exposure, such as a restaurant or type of food they’ve all eaten.