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FDA Commissioner Robert Califf is set to answer questions Wednesday from House lawmakers probing the events leading to the formula shortage, which has forced the U.S. to begin airlifting products from Europe while many parents still hunt for scarce supplies in stores.

The issue is largely tied to problems at Abbott Nutrition’s Michigan plant, the largest in the U.S., which the FDA shut down in February due to contamination. In prepared remarks, Califf gives the first detailed account of why it took his agency months to inspect the plant after first learning of potential problems last fall.

Members of an Energy and Commerce subcommittee will also hear from three infant formula makers, including a top executive from Abbott.

FDA staff began honing in on Abbott's plant last year while tracking four bacterial infections in infants who had consumed formula from the facility. The cases occurred between September and January, causing four hospitalizations, including two deaths.

Califf will tell lawmakers that the FDA began planning to visit the Sturgis, Michigan, plant in early December, with inspectors set to arrive on Dec. 30. But Abbott said that about a dozen of its employees had recently tested positive for COVID-19 and requested a delay. As a result, the FDA didn't begin its inspection until Jan. 31.