In this Aug. 22, 2017 photo, White House Chief of Staff John Kelly and Deputy Chief of Staff Kirstjen Nielsen speak together as they walk across the South Lawn of the White House in Washington. President Donald Trump is expected to nominate Kirstjen Nielsen as his next Secretary of Homeland Security. That’s according to three people familiar with decision. They spoke on condition of anonymity in order to discuss deliberations before a formal announcement. Nielsen was former DHS Secretary John Kelly’s deputy when he served in that role and moved with Kelly to the White House when he was tapped to be Trump’s chief of staff. (AP Photo/Andrew Harnik)

In this Aug. 22, 2017 photo, White House Chief of Staff John Kelly and Deputy Chief of Staff Kirstjen Nielsen speak together as they walk across the South Lawn of the White House in Washington. President Donald Trump is expected to nominate Kirstjen Nielsen as his next Secretary of Homeland Security. That’s according to three people familiar with decision. They spoke on condition of anonymity in order to discuss deliberations before a formal announcement. Nielsen was former DHS Secretary John Kelly’s deputy when he served in that role and moved with Kelly to the White House when he was tapped to be Trump’s chief of staff. (AP Photo/Andrew Harnik)

WASHINGTON (AP) — President Donald Trump will nominate his deputy chief of staff, Kirstjen Nielsen, as his next secretary of Homeland Security, the White House announced Wednesday.

Nielsen formerly served as John Kelly’s chief of staff when he held the position of Trump’s first DHS secretary. She moved with Kelly to the White House when Trump tapped him to serve as his own chief of staff, and was quickly named principal deputy.

Nielsen, an expert in homeland and national security policy, previously served as a special assistant to former President George W. Bush and worked for the Transportation Security Administration.

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The White House said in its announcement that Nielsen “has extensive professional experience in the areas of homeland security policy and strategy, cybersecurity, critical infrastructure, and emergency management.” She is also the first nominee for the position to have previously worked for DHS, they said.

Nielsen, however, has drawn some resentment in her current position, as she and Kelly have tried to impose order on a formally chaotic White House. That has included limiting access to the president to some who had formerly enjoyed near-unimpeded access.

Elaine Duke has been filling in as acting secretary of the department since Kelly’s departure, leading the department through a series of back-to-back hurricanes, including one that devastated most of Puerto Rico. But she raised eyebrows when she described that storm’s aftermath as a “good news story.”

Rep. Bennie G. Thompson, D-Miss., the ranking member of the Committee on Homeland Security, said in a statement he was pleased the president had made a decision on filling the post “after letting the critical national security position remain vacant as the nation faced multiple major hurricanes and a domestic terrorism attack.”

But Thompson said he had questions about her background, including her past work for the Bush administration.

Nielsen must be confirmed by the Senate.

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Associated Press writers Catherine Lucey, Jonathan Lemire and Ken Thomas contributed to this report.