Investigators in the US who had eventually traced a nationwide outbreak of food poisoning to a Pennsylvania factory have now discovered the same strain of potentially deadly bacteria at a poultry plant 50 kilometres away in New Jersey, according to a report from Associated Press.

The national Centers for Disease Control said last month that the Wampler Foods plant had been responsible for a listeria outbreak that killed seven people and sickened 50. But last week, a batch of listeria genetically indistinguishable from that found at the Wampler plant turned up in a sample of deli meat at another food factory, J.L. Foods in New Jersey.

CDC spokesman Tom Skinner told AP that authorities are investigating whether "a common source product", such as the same batch of raw turkey meat or else the possibility of tainted packaging materials, which might have been used at both plants.

The discovery now raises doubts as to whether the Pennsylvania plant was at fault itself.

"We believe that this finding suggests that the outbreak may have another source, and this is consistent with what we have been saying all along," Skinner told AP.