Senior congressional leaders have expressed concerns to the defense secretary about the status of the office tasked with overseeing US special operations.

The letter outlining their concerns was sent the same day that the acting official overseeing that office announced his resignation.

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The chairmen and ranking members of the House and Senate Armed Services Committees sent a letter to Defense Secretary Mark Esper on Wednesday expressing concern about staffing vacancies in the office overseeing special operations — the same day that the head of that office resigned.

Mark Mitchell, acting assistant defense secretary for special operations and low-intensity conflict, submitted his resignation "as of November 1," as first reported by CNN.

Mitchell has been the Pentagon's top special operations official since June 22, when his predecessor, Owen West, resigned to spend more time with his family. West held the job for just over 18 months.

The reason for Mitchell's departure was not immediately clear. He will be replaced by Thomas Alexander, who currently runs Defense Department counternarcotics efforts.

Mitchell's departure adds to a leadership vacuum described by House Armed Services Committee chairman Rep. Adam Smith and ranking member Rep. Mac Thornberry and Senate Armed Services Committee chairman Sen. James Inhofe and ranking member Sen. Jack Reed.

US airmen from the 320th Special Tactics Squadron listen to details for an upcoming mission from their team lead, at Camp Hansen, Japan, November 19, 2015. US Air Force photo/Senior Airman John Linzmeier

Their letter, provided to Business Insider, notes reforms included in the 2017 National Defense Authorization Act "designed to enhance the role of the Assistant Secretary of Defense for Special Operations and Low Intensity Conflict [ASD SOLIC] in providing for civilian oversight and advocacy of Special Operations Forces."

But a May 2019 report by the Government Accountability Office cited in the letter found that almost all the remaining tasks related to those reforms "did not have clear time frames for implementation" and that "current guidance" about the assistant secretary's responsibilities was "outdated."

The lawmakers said their biggest concern is the "lack of progress in staffing" the Special Operations secretariat.

A manpower survey commissioned by the assistant secretary's office found that up to 64 full-time jobs may be needed. But as of October 1, 2018, according to the letter, the secretariat had 22 personnel, and by the end of fiscal year 2019 on September 30, it had added just six personnel, "despite clear congressional direction," the letter says.

"We are concerned that the factors described above and that lack of a confirmed ASD SOLIC, Principal Deputy ASD SOLIC, or Director of the Secretariat are significantly inhibiting the ability of the office of the ASD SOLIC to fulfill its statutorily director responsibilities," the committee members wrote, asking Esper to "personally look into the issues" and provide "a plan and associated timeline" for clarifying guidance, hiring staffers, and completing other actions to ensure the assistant secretary can do its job.

The 'SOF easy button'

Mitchell fires a Humvee-mounted MK-19 grenade launcher alongside a Special Tactics Airman with the 26th Special Tactics Squadron at the Eglin Range, Florida, November 30, 2017 US Air Force/Airman 1st Class Joseph Pick

The focus on special operations oversight comes amid increased use of those forces and growing concern about the effect of that use on their effectiveness.

During a House hearing in May 2017, Theresa Whelan, then principal deputy assistant secretary of defense for special operations, said Special Operations Command has had to "essentially ... eat our young" because of the demands of a high operational tempo at a time of declining budgets.

"We've mortgaged the future in order to facilitate current operations. That has impacted readiness, and it's also impacted development of force for the future," she said. (The 2017 NDAA containing the reforms described in the letter was passed in December 2016.)

Mitchell, himself a former Army special operator, also expressed concerns about that overreliance — what he called the "SOF easy button."

"There's a tendency amongst some policy makers and some leaders—both civilian and military—to look to Special Operations to solve hard problems. And while we're very good at doing that, not every hard problem has a good SOF solution," he said in a 2018 interview with West Point's Combating Terrorism Center. "Special Operations, while they are an important part of achieving our national security objectives, can very rarely be the sole solution."