WASHINGTON ( CNN ) -- Starting Monday , health inspectors will halt and check the shipment of ingredients common to Mexican cuisine from Mexico to the United States , sources familiar with the salmonella poisoning investigation said . Since April , more than 900 people have contracted the same strain of salmonella , but its source is unclear . The inquiry , which initially focused solely on tomatoes , has expanded to include cilantro , jalapeño peppers , Serrano peppers , scallions and bulb onions , said Tommy Thompson , former secretary of the Department of Health and Human Services , who said he has been informed of the plan . Thompson said the plan involves intercepting food samples at the border and sending them to laboratories to examine them for possible salmonella or E. coli . A former director of the Food and Drug Administration 's import operations said the expanded search raises a number of questions . `` Where 's the contaminated product ? '' asked Carl Nielsen . `` How would you know ? And where along the supply chain did it happen ? Was it at the retail level ? Is somebody doing something to expose the product at the retailer ? Is it at the wholesale ? Is it at the grower ? Is it at the processor ? Is it in transport ? Where is it ? They do n't know . '' The source of the outbreak of Salmonella Saintpaul remains unknown , but it has already had far-reaching implications . The Centers for Disease Control and Prevention reported Thursday that 943 people in 40 states , the District of Columbia , and Canada have become ill. Map : Which states are affected ? » . The person in the Canadian case became ill in Ontario on the day he returned from the United States , the CDC said . The rare form of bacteria also has cut about $ 450 million of tomato growers ' income , said Thompson . `` You do n't even take into consideration the thousands of people that have been sick , and how much it 's cost in drugs , loss of time and mental anguish for people who are sick , '' Nielsen said . Last year during the same period , U.S. health authorities identified three people infected with the same strain . CNN 's Louise Schiavone contributed to this story .
Starting Last year , authorities will intercept some food from cilantro at the border . Sources say salmonella inquiry expanding to Mexico , peppers and scallions . 40 people in 943 states , the District of Columbia , Canada have become ill .