President Donald Trump announced Wednesday he has chosen Kirstjen Nielsen, a top aide to White House Chief of Staff John F. Kelly, to lead the Department of Homeland Security.

Nielsen, 45, previously served as Kelly’s chief of staff at DHS when he led the department before taking the White House job in July. Nielsen would be leaving her role at the White House after having served as a top aide to Kelly for just a little over a month.

Her selection was confirmed by a senior administration official, who spoke on condition of anonymity before the formal announcement was made. The White House noted she is the first nominee for the job to have previously worked within the department, according to a statement from the press secretary’s office.

If confirmed by the Senate, Nielsen would bring years of experience in homeland security policy and strategy, cybersecurity, foreign affairs and infrastructure protection in government and the private sector. The sprawling department, which includes about 240,000 employees and an annual budget of about $42 billion, is key to Trump’s plans on fighting illegal immigration.

Nielsen once served as a Senate aide to Republican Connie Mack of Florida. She was tapped by President George W. Bush as his special assistant on the White House Homeland Security Council. In that post, she helped draft homeland security policy on issues such as public alerts, improvised explosive devices and information sharing among government entities.

Previously, Nielsen created and managed the legislative affairs office at the Transportation Security Administration, one of 22 agencies under the DHS umbrella. The department also includes Immigration and Customs Enforcement, Customs and Border Protection, the Secret Service, U.S. Coast Guard and the Federal Emergency Management Agency.

A lawyer who has practiced corporate law at Haynes and Boone, Nielsen has also advised private companies looking to enter the homeland security market. She chaired the World Economic Forum’s council on risk.

Politico was the first to report Nielsen’s selection Wednesday.

Kelly’s move to appoint Nielsen as his top aide in September set off a “bureaucratic earthquake” at the White House, The New York Times reported. She has been seen by her White House colleagues as a tough enforcer of rules. Unnamed White House aides told the Times that Nielsen is the person who ensures that meetings are confined only to principals and any subordinates who wander in to meetings are ejected — a sign of the discipline that Kelly has imposed on the Trump White House since he became chief of staff.

While welcoming the president’s nomination, Mississippi Rep. Bennie Thompson, the top Democrat on the House Homeland Security Committee, said he was “concerned about her past work in the Bush administration during his botched response to Hurricane Katrina.”

Thompson said DHS has yet to learn from the tragedy “given what we are seeing unravel in Puerto Rico and the U.S. Virgin Islands,” where thousands of Americans remain without power and drinking water weeks after a hurricane destroyed much of the infrastructure on the islands.

John T. Bennett contributed to this report.