State health officials warned consumers not to drink milk produced by a Massachusetts dairy, saying it was the likely source of a bacterial illness that killed two men and sickened two others.

Whittier Farms suspended operations and distribution, said Dr. Alfred DeMaria, the state director of communicable disease control. Most of its customers live in Worcester County.

The Department of Public Health issued the warning Thursday after identifying four cases of listeriosis. Two of the victims, a 78-year-old man and a 75-year-old man, died in June and October.

Investigators have not identified the exact source of the contamination in the production process, DeMaria said. But milk samples gathered last week at one of the dairy's two retail stores tested positive for listeria.

Testing showed that the bacteria causing the infections in the four victims likely came from the same source.

Listeriosis is a kind of food poisoning that can be dangerous to the elderly, newborns, pregnant women and people with chronic medical conditions. Symptoms include fever, headache, stiffness, nausea, abdominal pain and diarrhea.

Officials at Whittier Farms did not immediately return calls Thursday seeking comment.

The dairy has a processing plant in Shrewsbury and operates two retail stores. A message left on an answering machine at the Sutton store said it was closed and that information to date was "inconclusive." The message said the owners would be addressing the issue as soon as possible.

Whittier Farms' milk products are sold under several brand names, including Whittier, Schultz, Balance Rock, Spring Brook, and Maple, state health officials said.