President Donald Trump’s administration has reportedly fired an unprecedented number of high-level State Department employees in a major shakeup at the federal agency.

“It’s the single biggest simultaneous departure of institutional memory that anyone can remember, and that’s incredibly difficult to replicate,” David Wade, who served as State Department chief of staff under Secretary of State John Kerry, told the Washington Post, which first broke the story.

“Department expertise in security, management, administrative and consular positions in particular are very difficult to replicate and particularly difficult to find in the private sector,” he added.

As former ExxonMobil CEO Rex Tillerson, Trump’s pick to helm the State Department, was getting acquainted with Foggy Bottom this week, there was a smattering of firings and resignations occurring.

While Post reporter Josh Rogin’s report suggested that all the officials resigned, CNN has a different take. State Department officials told Elise Labott, CNN’s global affairs correspondent, that the Trump White House fired four top officials.

.@eliselabottcnn reports they were sent letters by the White House that their service was no longer required — Jim Sciutto (@jimsciutto) January 26, 2017

The officials who are out, either through firings or resignation, are Undersecretary for Management Patrick Kennedy, Assistant Secretaries of State Joyce Anne Barr, Gregory Starr and Michele Bond, Director of Foreign Missions Gentry Smith and Director of the Bureau of Overseas Building Operations Lydia Muniz.

And according to Wade, there is just no easy way to fill those posts again:

Diplomatic security, consular affairs, there’s just not a corollary that exists outside the department, and you can least afford a learning curve in these areas where issues can quickly become matters of life and death. The muscle memory is critical. These retirements are a big loss. They leave a void. These are very difficult people to replace.

The Post report expressed uncertainty about whether or not Kennedy, who had previously been angling to keep his job under Tillerson, was among those fired, given the issue seems to be disputed among State Department staffers.

Kennedy aided a lot with the transition, according to the report, but he has a few skeletons in his closet leftover from when former Secretary of State Hillary Clinton was leading the agency.

According to Politico, Kennedy was instrumental in trying to coax the FBI into changing its mind about giving one of Clinton’s emails a secret classification during its investigation:

Deputy Secretary of State for Management Patrick Kennedy intervened with the FBI to dispute the classification at least three times: in a May 14, 2015, call to International Operations Division chief Brian McCauley, at an in-person meeting at the State Department five days later and in a phone conversation with the head of FBI’s Counterterrorism Division, Michael Steinbach. The unnamed FBI author of the message to Jupina said Kennedy summoned various officials to State to discuss the review of 55,000 of Clinton emails requested under FOIA. At that meeting, Kennedy asked the FBI representative and a Justice Department FOIA official to “stay behind to discuss the FBI determination” on classification in the first batch of Clinton emails, the FBI email says. An email from Steinbach said he turned down Kennedy’s request that the information be withheld solely under a FOIA provision for protection of law enforcement sources, rather than by classifying it.

His efforts, however, proved unsuccessful.

Trump vowed on the campaign trail to “drain the swamp,” but it remains to be seen if this is exactly the kind of draining he had in mind.

In the election, Trump constantly railed against the former President Barack Obama’s approach to foreign policy and promised to upend many of the practices he had put in place, most notably when it comes to defeating ISIS. But with no governmental experience of his own, the vacancies could pose a big problem for Trump’s administration.

Upon confirmation, Tillerson’s most pressing job will be to find qualified and experienced personnel to fill the new void.

“You don’t run foreign policy by making statements, you run it with thousands of people working to implement programs every day,”Ambassador Richard Boucher, who served as State Department spokesman for Colin Powell and Condoleezza Rice, told the Post. “To undercut that is to undercut the institution.”