Listeria, a deadly bacterium that killed three people and sickened two others, has been found in additional samples of milk products and several environmental samples taken at the Whittier Farms milk processing plant on January 2 and 3, public health officials said Thursday.

Tests performed at the plant found a strain of listeria on the floor that was identical to the type found in people who became ill last year after consuming the dairy’s milk. Investigators discovered the bacterium near a key piece of equipment used after milk was pasteurized.

The same strain was identified in seven unopened containers of milk, which were removed from a retail adjacent to the Whittier Farms production facilities. It was found in skim milk as well as coffee-, chocolate-, vanilla- and strawberry-flavored milk.

Investigators do not kow for sure how listeria made its way inside the plant. The Whittier plant has been appreciated for the good quality of its products. According to Suzanne Condon, the top environmental health official at the state Department of Public Health, workers might have brought the bacterium from elsewhere on their clothing or shoes.

"We know that there’s a problem in that plant and we have connected the patients to the products to the plant, not it would be nice to know exactly how that happened, but that is part of the ongoing investigation," the Associated Press quoted Dr. Alfred DeMaria, state director of communicable disease control.

So far, three elderly men have died since June from drinking listeria-contaminated milk from the Whittier Farms plant in Shrewsbury, about 35 miles west of Boston. The bacterium also sickened a pregnant woman, who then miscarried. A second woman also was sickened after drinking milk from the plant.

The pant was closed in December. It will remain closed during the investigation and until the listeria is eradicated, officials said.

Symptoms of listeriosis, the disease caused by the deadly bacterium, include fever, headache, stiffness, nausea, abdominal pain and diarrhea and can lead to the death of babies and people with weakened immune systems.