WASHINGTON (AP) — Federal plans to inspect a baby formula factory linked to the nationwide shortage were slowed by COVID-19, scheduling conflicts and other logistical problems, according to prepared testimony from the head of the Food and Drug Administration.

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.

The problem 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 manufacturers, including a top Abbott executive.

FDA staff began honing in on problems at Abbott’s plant after 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.

After detecting positive samples of bacteria in multiple parts of the plant, the FDA closed the facility and Abbott announced a massive recall of its formula on Feb. 17.

Abbott and the FDA have reached an agreement to reopen the plant next week, requiring the company to regularly undergo outside safety audits. But Califf’s testimony suggests FDA efforts to reopen the plant were slowed by negotiations with Abbott, which had to be codified in a court agreement.

“Because it was a negotiation process with a regulated firm, the U.S. government did not completely control the timeline,” states Califf’s written testimony.

The FDA has also faced questions about its timeline for reviewing an October whistleblower complaint alleging numerous safety violations at Abbott’s plant, including employees falsifying records and failing to test formula. Califf’s testimony details a two-month gap between when regulators received the report and when they actually interviewed the whistleblower.

Several FDA staffers reviewed the complaint in late October, but officials didn’t request an interview until early December. Because of conflicts with the whistleblower’s schedule, the interview didn’t take place until Dec. 22, according to the FDA testimony.

Senior FDA officials did not receive copies of the whistleblower complaint until February due to “an isolated failure in FDA’s mailroom, likely due to COVID-19 staffing issues,” according to the prepared remarks.

Califf is the only administration official who has testified thus far on the shortage, which has become a major political liability for President Joe Biden. Behind the shortage are other distinct factors, including supply disruptions caused by COVID-19 and industry consolidation that’s made the U.S. formula market vulnerable to disruption.

An Abbott executive is expected to tell the committee that his company will invest in additional capacity and supply chain safeguards to avoid future disruptions. After the company restarts production next month, it will be able to produce more formula than before the recall, according to prepared remarks from Abbott’s senior vice president, Christopher Calamari.

The company will restate its contention that there is no direct link between its formula and the infant infections investigated by the FDA. Agency regulators have said the small number of cases and incomplete testing data make it hard to draw a direct connection between the illnesses and Abbott’s plant.

Executives from Reckitt and Gerber are also scheduled to testify.