There are now 19 known cases of salmonella infection linked to ground beef sold at Hannaford Supermarkets.

The U.S. Centers for Disease Control and Prevention released additional information today about a salmonella outbreak first reported on Dec. 15, when Hannaford issued a recall of ground beef with sell-by dates of Dec. 17 or earlier.

The U.S. Department of Agriculture is conducting an investigation into the source of  the contamination.

The number of reported cases of a rare strain of salmonella typhimurium is now 19 — up from 14 known cases on Dec. 15.

The number of Maine cases hasn’t risen from the four initially reported. The other 15 cases were reported in six states. New Hampshire had the most with six, followed by five in New York. One case was reported in each Vermont, Massachusetts, Kentucky and Hawaii.

The CDC said 14 of the infected people reported eating ground beef in the week before their illnesses began and 12 of them reported buying that beef from Hannaford stores between Oct. 12 and Dec. 10.

The CDC knows that at least seven of them were hospitalized. None have died.