WASHINGTON (CNN) -- Starting Monday, health inspectors will halt 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.

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