Nielsen, 45, is a cybersecurity expert and an attorney with an extensive background in homeland security, including stints at the Transportation Security Administration and on the White House Homeland Security Council under President George W. Bush.

“She would be the first person to run the department who has actually worked there,” said a person close to the administration. “She has a deep familiarity.”

Nielsen developed a close working relationship with Kelly during the transition, in which she served as his “sherpa,” guiding him through the Senate confirmation process.