WASHINGTON (AP) — 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 mid-air 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.

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

The employees were fired “without cause nor based on performance or conduct,” Spero said, and the emails were “from an ‘exec order’ Microsoft email address” — not a government email address.

The firings hit the FAA when it faces 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.

One FAA employee who was fired over the weekend suggested he was targeted for his views on Tesla and X, formerly Twitter, not as part of a general probationary-level sweep. Both are owned by Elon Musk, who is leading Trump’s effort to cut the federal government.

Charles Spitzer-Stadtlander posted on LinkedIn that he was fired just after midnight Saturday, days after he started getting harassing messages on Facebook.

“The official DOGE Facebook page started harassing me on my personal Facebook account after I criticized Tesla and Twitter,” Spitzer-Stadtlander wrote. “Less than a week later, I was fired, despite my position allegedly being exempted due to national security.”

He added: “When DOGE fired me, they turned off my computer and wiped all of my files without warning.”

Spitzer-Stadtlander said he was supposed to be exempted from the probationary firings because the FAA office he worked in focused on national security threats such as attacks on the national airspace by drones.

The Musk-led Department of Government Efficiency did not immediately respond to a request for comment. The firings were first reported by CNN.

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Associated Press writer Ellen Knickmeyer contributed from Washington.