Trump administration officials fired more than 300 staffers on Thursday night at the National Nuclear Security Administration — the agency tasked with managing the nation’s nuclear stockpile — as part of broader Energy Department layoffs, according to four people with knowledge of the matter.

Sources told CNN the officials did not seem to know this agency oversees America’s nuclear weapons.

An Energy Department spokesperson disputed the number of personnel affected, telling CNN that “less than 50 people” were “dismissed” from NNSA, and that the dismissed staffers “held primarily administrative and clerical roles.”

The agency began rescinding the terminations Friday morning.

Some of the fired employees included NNSA staff who are on the ground at facilities where nuclear weapons are built.

These staff oversee the contractors who build nuclear weapons, and they inspect these weapons.

It also included employees at NNSA headquarters who write requirements and guidelines for contractors who build nuclear weapons.

A source told CNN they believe these individuals were fired because “no one has taken anytime to understand what we do and the importance of our work to the nation’s national security.”

Members of Congress made their concerns about the NNSA firings known to the Energy Department, a Hill staffer told CNN.

A person with knowledge of the matter told CNN that senators visited Energy Sec. Chris Wright to express concern about the NNSA cuts.

“Congress is freaking out because it appears DOE didn’t really realise NNSA oversees the nuclear stockpile,” one source said.

“The nuclear deterrent is the backbone of American security and stability – period. For there to be any even very small holes poked even in the maintenance of that deterrent should be extremely frightening to people.”

NNSA has a total of 1800 staff at facilities around the country.

The only probationary staffers exempt from the Thursday-night firings were those who work at its Office of Secure Transportation, the office in charge of driving or otherwise transporting nuclear weapons around the country securely, one person familiar told CNN.

“There is strong support on the Hill for NNSA in nuclear modernisation writ large,” one source told CNN.

“Clearly, NNSA is a critical agency. There have been lawmakers with concerns.”

The agency made the about face Friday morning; during a meeting, acting NNSA administrator Teresa Robbins said the agency had received direction to rescind the termination of probationary employees.

Probationary workers have typically been employed for less than a year, or two years in some cases, and have fewer job protections and rights to appeal.

Robbins added on Friday that if probationary NNSA employees had not yet been fired, their jobs were now safe and all NNSA employees whose access to the agency’s network and internal IT systems was shut off would be turned back on, one source told CNN.

The source said Robbins added, “There is a good probability that most or all probationary employees who were fired could return.”

Another source cautioned the situation was extremely fluid and said “we don’t know” how many people will be returning.

An NNSA spokesperson referred CNN’s questions to DOE.

“The Energy Department will continue its critical mission of protecting our national security and nuclear deterrence in the development, modernisation, and stewardship of America’s atomic weapons enterprise, including the peaceful use of nuclear technology and nonproliferation,” the DOE spokesperson told CNN.

Political officials at the Energy Department told its non-political HR administrators to cite poor performance personnel files as a justification for firing the employees, the source said.

Frustrated by the pressure from political appointees, two of those HR employees submitted their resignations on Friday.

A DOE spokesperson declined to comment on the poor-performance rationale for the firings. CNN has reached out to the two employees who resigned.

In addition to overseeing America’s nuclear weapons, the NNSA also helps secure nuclear material nationwide.

Sources told CNN it’s a critical mission, pointing to the Russian drone attack on a Chernobyl power plant reactor in Ukraine on Thursday.

“NNSA maintains sensors in Ukraine to help track nuclear risks, whether intentional or unintentional,” a source said, adding the layoffs are “frightening.”