"Several hundred" Federal Aviation Administration employees have received termination notices, according to their union and another source inside the FAA.

David Spero, the national president of the Professional Aviation Safety Specialists, said in a statement that the organization was "troubled and disappointed by the administration's decision to fire FAA probationary employees PASS represents without cause nor based on performance or conduct."

Spero said "several hundred employees" received messages from an "exec order" Microsoft email address -- not an official .gov email address -- from 7 pm ET on Feb. 14.

"These are not nameless, faceless bureaucrats," Spero said. "They are our family, friends and neighbors. They contribute to our communities. Many military veterans are among them. It is shameful to toss aside dedicated public servants who have chosen to work on behalf of their fellow Americans."

"These employees were devoted to their jobs and the safety critical mission of the FAA," Spero continued. "This draconian action will increase the workload and place new responsibilities on a workforce that is already stretched thin."

"This decision did not consider the staffing needs of the FAA, which is already challenged by understaffing," he added.

"Staffing decisions should be based on an individual agency's mission-critical needs," Spero said. "To do otherwise is dangerous when it comes to public safety. And it is especially unconscionable in the aftermath of three deadly aircraft accidents in the past month."

-ABC News' Sam Sweeney

Secretary of State Marco Rubio arrived in Saudi Arabia on Monday ahead of expected U.S.-Russia talks there on ending Moscow's war on Ukraine, though Kyiv's representatives are not expected to take part.

Rubio will be joined by Middle East envoy Steve Witkoff and National Security Adviser Mike Waltz.

Kremlin spokesperson Dmitry Peskov told reporters on Monday that the Russian delegation would include Foreign Minister Sergei Lavrov, President Vladimir Putin's aide Yuri Ushakov and Kirill Dmitriev -- the head of Russia's sovereign wealth fund.

"On Tuesday in Riyadh they will hold a meeting with American counterparts, which will be devoted, first of all, to restoring the entire range of Russian-American relations," Peskov said. The teams will also discuss a potential in-person meeting between Putin and President Donald Trump, Peskov said.

Ukraine is not expected to be represented in the coming round of talks, despite calls from Kyiv and across Europe for President Volodymyr Zelenskyy and his team to play a role.

-ABC News' Joe Simonetti

President Donald Trump's administration acknowledged on Sunday night that some federal government employees who took the "Fork in the Road" buyout offer were also, subsequently, fired or let go — and that this was an error.

An Office of Personnel Management official told ABC News that some employees who responded to the buyout offer ahead of the deadline last week may have received termination notices by mistake but, for those personnel, the buyouts agreements would be honored.

Nick Detter told ABC News that he is one of those workers. Detter, a natural resource specialist with the USDA, said he was fired Thursday even though he already accepted the administration's buyout offer, meaning that he should have been paid through September.

Despite OPM’s explanation, Detter says he hasn’t been able to get any guidance directly. He said that his supervisors in Kansas, where he’s based, told him they have no information.

"I frankly find it pretty insulting and chaotic and disorganized," Detter told ABC News.

"I would never say that there's no room for improvement efficiency in the federal government," he said. "But in my experience over the last month with this whole thing, that's not what this has been. This has just been slash and burn."

-ABC News' Rachel Scott, Cheyenne Haslett, Sarah Lang and Ariana Nalty

Elon Musk’s Department of Government Efficiency has requested access to an IRS data system that retains the personal tax information on millions of Americans, two sources familiar with the matter told ABC News.

The system, known as the Integrated Data Retrieval System, or IDRS, is used by IRS employees to review tax information, issue notices to taxpayers and update taxpayer records.

Access to the data, which is tightly controlled within the agency, had not been granted as of this weekend, several sources told ABC News.

-ABC News' Anne Flaherty, Soo Youn, Benjamin Siegel, Olivia Rubin and Hannah Demissie