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A top official at the Federal Emergency Management Agency resigned in December, four months after a background investigation derailed his nomination to lead the agency, according to a letter obtained Monday by NBC News.

In the Dec. 20 letter, the official, Jeffrey Byard said that he "had the privilege of leading the finest workforce during the most impactful natural disaster period in our nation's history."

The letter says Byard’s last day will be on February 1, 2020.

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Jeff Byard served as the Associate Administrator for the Office of Response and Recovery (ORR) at the Federal Emergency Management Agency (FEMA). FEMA

Byard served as the "senior-most executive over disaster response, recovery, logistics, and field operations" at FEMA and oversaw the agency during Puerto Rico's troubled recovery from the 2017 hurricanes.

President Donald Trump nominated Byard to run the agency last February, but the White House withdrew his nomination in September after it was held up during an investigation by the Senate Homeland Security and Governmental Affairs Committee.

There were 100 natural disasters declared by FEMA in the U.S. in 2019, down from 124 in 2018 and 137 in 2017.

A spokeswoman for the agency said Byard had served honorably for two years. He was leaving to pursue a position outside government, she said.

The Trump administration has been plagued with high-level Cabinet vacancies. A count by NBC News last May found that the president's nominees were being confirmed at a record slow pace.

According to an online tracker by The Washington Post, the administration has 168 key positions that are still without a nominee.

Observers of presidential appointments have attributed the vacancies to persistent turnover within the administration.

"There's still an element of musical chairs — filling one job and creating a vacancy somewhere else," said Max Stier, president and CEO of the Partnership for Public Service, a nonpartisan nonprofit that tracks presidential appointments. That causes "cascading effects. Somebody leaves, others follow. You're creating disruptions throughout the whole organization."