Federal health officials have reversed the decision to fire a few dozen scientists at the Food and Drug Administration’s food-safety labs, and say they are conducting a review to determine if other critical posts were cut.

A spokesman for the Department of Health and Human Services confirmed the rehirings and said that several employees would also be restored to the offices that deal with Freedom of Information requests, an area that was nearly wiped out.

In the last few months, roughly 3,500 F.D.A. jobs, about 20 percent, were eliminated, representing one of the largest work force reductions among all government agencies targeted by the Trump administration.

The H.H.S. spokesman said those employees called back had been inadvertently fired because of inaccurate job classification codes.

The decision to rehire specialists on outbreaks of food-related illnesses and those who study the safety of products like infant formula follows contradictory assertions made by Dr. Marty Makary, the F.D.A. commissioner, in media interviews this week.

“I can tell you there were no cuts to scientists or inspectors,” Dr. Makary said Wednesday on CNN.

In fact, scientists had been fired from several food and drug safety labs across the country, including in Puerto Rico, and from the veterinary division where bird flu safety work was underway. Scientists in the tobacco division who were dismissed in February — including some who studied the health effects of e-cigarettes — remain on paid leave and have not been tapped to return, according to employees who were put on leave.

How many fired employees will be permitted to return remained unclear.

About 40 employees at the Moffett Lab in Chicago and at a San Francisco-area lab are being offered their jobs back, the department spokesman said. Scientists in those labs studied a variety of aspects of food safety, from how chemicals and germs pass through food packaging to methods for keeping bacteria out of infant formula. Some scientists in Chicago reviewed the work and results of other labs to ensure that milk and seafood were safe.

Dr. Robert Califf, the F.D.A. commissioner under President Joseph R. Biden, said the terms “decapitated and eviscerated” seemed fitting to describe the steep loss of expertise at the agency. He said the F.D.A. was already falling behind on meetings meant to help companies develop safe products — and to design studies that give clear answers about their effectiveness.

“Most of it is really at this level of fundamental, day-to-day work that has a huge impact overall, but it’s not very controversial,” he said. “It’s just that it takes work, and they have to have people to do the work.”

Dr. Makary has also said the layoffs did not target product reviewers or inspectors. But their work has been hampered by voluntary departures, the elimination of support staff and the broader disruption at an agency where many are fleeing for the exits, according to former staff members.

Hundreds of drug and medical device reviewers, who make up about one-fourth of the agency work force, have recused themselves from key projects, Dr. Scott Gottlieb, a former agency commissioner, said on CNBC. Under F.D.A. ethics rules, staff members who are interviewing for jobs cannot do agency review work on products by companies where they are seeking employment — or for a competitor.

Dr. Gottlieb also said cuts to the office of generic drug policy wiped out employees with expertise in determining which brand-name drugs are eligible to be made as lower-cost generics, calling those job eliminations “profound.” Approving generic drugs can save consumers billions of dollars.

Support staff for inspectors investigating food and drug plants overseas were also cut, raising security concerns. Dozens of workers who lost their jobs attended to security monitoring to ensure that inspectors were safe, especially in hostile nations.