The head of the State Department’s diplomatic security bureau has resigned and will leave his post on Thursday, leaving the two top positions vacant for the foreseeable future in the bureau in charge of embassy security around the world.

Acting Assistant Secretary of State for Diplomatic Security Bill Miller announced his resignation and retirement from the State Department on July 19 in a note to staff, which I obtained. His last day of service will be July 27. Miller has been the top official in the Diplomatic Security Service since January, when the Senate-confirmed assistant secretary Gregory Starr was asked to resign.

Starr had been singled out by Republicans during the investigation of the Sept. 11, 2012, attack on the U.S. diplomatic mission in Benghazi, Libya. Starr was asked to resign after President Trump’s inauguration, along with a host of other senior State Department officials. This is common at the beginning of a new administration, although traditionally officials are not pushed out until replacements are found.

Miller, whose official title was principal deputy assistant secretary of state and director of the Diplomatic Security Service, resigned voluntarily and was not asked to resign or fired, a senior State Department official told me. He has been a member of the Diplomatic Security Service since 1987.

“It is with extraordinary sadness, counterbalanced by extreme pride, and with the deepest sense of humility, that I announce my retirement from the Department of State and the Diplomatic Security Service,” he told colleagues in his note. “I have been blessed throughout my life in so many ways. I count among those blessings the opportunity to SERVE with all of you.”

Christian Schurman, the deputy assistant secretary for international programs, will assume the role of acting assistant secretary for the bureau, said Heather Nauert, State Department spokeswoman.

“We thank [Miller] for his leadership and dedicated service, and we wish him the very best,” she said.

Sources close to Miller told me that he had been hoping to be appointed to lead the Diplomatic Security Service on a permanent basis but was informed that he would not be getting the job. Two sources familiar with the situation said that Secretary of State Rex Tillerson is planning to bring on Michael Evanoff, who was the head of Secretary of State Condoleezza Rice’s personal security detail when she led Foggy Bottom.

Rice has been advising Tillerson and was part of the effort to persuade President Trump to choose him for secretary of state. Evanoff declined to comment.

Sen. Bob Corker (R-Tenn.) says he views Secretary of State Rex W. Tillerson as "a patriot,” noting however, that there is a "dissonance" between President Trump and Tillerson. (Washington Post Live)

Miller’s departure is only the latest in a string of senior State Department officials who have decided to resign or retire since Trump came into office. Acting Assistant Secretary of State for Near Eastern Affairs Stuart Jones will retire early next month and move to the Cohen Group, a D.C. consulting firm.

Several other senior State Department officials and Foreign Service officers have decided to retire rather than take on new posts. And while turnover is natural with any change of administration, several officials also say that there is broad frustration in the department’s senior ranks with Tillerson and his approach to running the department.

“If no one is going to give you the job but they are going to ask you to crush your team with budget cuts and impossible tasks, why would you stay?” said one recently departed senior State Department official. “The thing this administration doesn’t get about the foreign service and State in general is that our skills and expertise are very marketable in the private sector. We have choices.”

Besides Tillerson, only one senior State Department political appointee and a handful of ambassadors are in place. Zero assistant secretaries of state have been confirmed. Tillerson has reportedly clashed with the White House on personnel issues, including his initial desire to appoint Susan Thornton, the acting assistant secretary of state for East Asian and Pacific affairs, to serve permanently.

BuzzFeed reported that Tillerson is now favoring former Treasury Department official Olin Wethington for that post. Administration officials told me the White House prefers Michael DeSombre, a mergers and acquisitions attorney in Hong Kong who is “Worldwide President” of “Republicans Overseas.”

The diplomatic security post is particularly important because of the crucial role that bureau plays in protecting American officials and property at diplomatic posts all over the world. During the campaign, Trump often criticized Hillary Clinton for failing to protect diplomats as secretary of state, especially in the case of the Benghazi attack.

In addition to protecting U.S. diplomats and embassies in more than 160 countries, the diplomatic security bureau also protects the secretary of state and more than 100 domestic State Department facilities, investigates passport and visa fraud, conducts personnel security investigations and issues security clearances.

Even if Evanoff is nominated soon, it may take weeks or months for him to be confirmed by the Senate. Meanwhile, the office that is in charge of preventing the next Benghazi attack will have to make do without permanent leadership in place.