WASHINGTON (AP) — Federal efforts to inspect a baby formula factory linked to the nationwide shortage were delayed by COVID-19, scheduling conflicts and even a missing piece of mail, 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 in stores.

Califf will lay out a series of setbacks that slowed FDA’s response by several months, including a COVID-19 outbreak at the plant and a whistleblower complaint that didn’t reach FDA leadership due to a “mailroom failure.”

The shortage 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 learning of potential problems as early as September.

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 fall while tracking several bacterial infections in infants who had consumed formula from the facility. The four cases occurred between September and January, leading to hospitalization and 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 over delays in reviewing an October whistleblower complaint alleging numerous safety violations at Abbott’s plant, including employees falsifying records. 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 eventually received emailed copies of the complaint, but not until February due to “an isolated failure in FDA’s mailroom, likely due to COVID-19 staffing issues,” according to the prepared remarks. A mailed copy addressed to then-acting commissioner Dr. Janet Woodcock has still not been located, according to the FDA.

Political fury over the shortage has landed squarely on the FDA and Califf, the only administration official who has testified thus far on the issue. The problems have quickly snowballed into a political firestorm for President Joe Biden, who has invoked the Defense Production Act and emergency flights to shore up supplies.

Califf is certain to face questions about why his agency didn’t anticipate the shortage, given that Abbott’s plant supplies roughly one-sixth of the U.S. formula supply. FDA regulators did not contact the U.S. Department of Agriculture about impacts to the formula supply until Feb. 11. According to the testimony. Califf will make the case that his agency does not have the “authority, resources, or dedicated staff” to track supply chain disruptions. He will ask lawmakers for new powers and resources to monitor the information.

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.