Three elderly men have died and at least one pregnant woman has miscarried since last June after drinking bacteria-contaminated milk, but health officials investigating a Massachusetts dairy plant found nothing wrong with its pasteurization process.

Tests at the Whittier Farms dairy plant, in Shrewsbury, about 35 miles west of Boston, found nothing wrong with its pasteurization process and health investigators are now looking at the cooling and bottling machinery for the source of the bacteria, said Dr. Alfred DeMaria, state director of communicable disease control.

The outbreak is believed to be the third time listeria has ever been linked to pasteurized milk in the United States, he said, adding "We know something is going on; we just don't know what it is. We just need to find out how the bacteria is getting into the milk."

The dairy suspended operations until the source of the contamination is pinpointed, the Associated Press reported.