Health officials have determined that soy lecithin in some Hershey products tested positive for salmonella, but they still are unsure how the soy was contaminated.

Fears of salmonella contamination at the company’s factory in Smith Falls, Ontario prompted a recall last weekend of a variety of chocolate bars and candies. Hershey now says soy lecithin tested positive for salmonella contamination.

Soy lecithin is used to help chocolate flow more easily during manufacturing.

The company, which employs 500 workers in the eastern Ontario town, shut down production and issued a recall of 25 products after a routine inspection inside the plant detected salmonella on Nov. 9.

The Canadian Food Inspection Agency said the plant will remain closed until an investigation has been completed.