The Trump administration has begun firing several hundred Federal Aviation Administration employees, upending staff on a busy air travel weekend and just weeks after a January fatal midair collision at Ronald Reagan Washington National Airport.

Probationary workers were targeted in late-night emails Friday notifying them they had been fired, David Spero, president of the Professional Aviation Safety Specialists union, said in a statement.

The impacted workers include personnel hired for FAA radar, landing and navigational aid maintenance, one air traffic controller told The Associated Press. The air traffic controller was not authorized to talk to the media and spoke on condition of anonymity.

In a message posted to X late Monday, Transportation Secretary Sean Duffy said fewer than 400 FAA employees were fired and “Zero air traffic controllers and critical safety personnel were let go.”

A Transportation Department official told the AP earlier Monday that the agency has “retained employees who perform critical safety functions.” In a follow-up query the agency said they would have to look into whether the radar, landing and navigational aid workers affected were considered to handle critical safety functions.

The National Air Traffic Controllers Association said in a brief statement Monday it was “analyzing the effect of the reported federal employee terminations on aviation safety, the national airspace system and our members.”

Other FAA employees who were fired were working on an urgent and classified early warning radar system the Air Force had announced in 2023 for Hawaii to detect incoming cruise missiles, through a program that was in part funded by the Defense Department. It’s one of several programs that the FAA’s National Airspace System Defense Program manages that involve radars providing longer-range detection around the country’s borders.

Due to the nature of their work, staff in that office typically provide an extensive knowledge transfer before retiring to make sure no institutional knowledge is lost, said Charles Spitzer-Stadtlander, one of the employees in that branch who was terminated.

The Hawaii radar and the FAA defense program office working on it are “about protecting national security,” Spitzer-Stadtlander said. “I don’t think they even knew what NDP does, they just thought, oh no big deal, he just works for the FAA.”

“This is about protecting national security, and I’m scared to death,” Spitzer-Stadtlander said. “And the American public should be scared too.”

Spero said messages began arriving after 7 p.m. Friday and continued late into the night. More might be notified over the long weekend or barred from entering FAA buildings Tuesday, he said.

The firings hit the FAA as it is facing a shortfall in controllers. Federal officials have been raising concerns about an overtaxed and understaffed air traffic control system for years, especially after a series of close calls between planes at U.S. airports. Among the reasons they have cited for staffing shortages are uncompetitive pay, long shifts, intensive training and mandatory retirements.

In the Jan. 29 fatal crash between a U.S. Army Black Hawk helicopter and American Airlines passenger jet, which is still under investigation, one controller was handing both commercial airline and helicopter traffic at the busy airport.

Just days before the collision, President Donald Trump had already fired all the members of the Aviation Security Advisory Committee, a panel mandated by Congress after the 1988 PanAm 103 bombing over Lockerbie, Scotland. The committee is charged with examining safety issues at airlines and airports.

Spitzer-Stadtlander suggested he was targeted for firing for his views on Tesla and X, formerly Twitter, not as part of a general probationary-level sweep. Both companies are owned by Elon Musk, whose Department of Government Efficiency is leading Trump’s effort to cut the federal government.

Spitzer-Stadtlander is Jewish and was angered by Musk’s straight-arm gesture at Trump’s inauguration. On his personal Facebook page he urged friends to get rid of their Teslas and X accounts in response.

Spitzer-Stadtlander said that post drew the attention of a Facebook account labeled “Department of Government Efficiency,” which reacted with a laughing emoji. Soon after, he saw the same account reacting to much older posts through his personal Facebook feed.