Tom Bossert, U.S. President Donald Trump's homeland security adviser, resigned on Tuesday, as John Bolton, Trump's new national security adviser, is reshaping the national security team.
"The president is grateful for Tom's commitment to the safety and security of our great country," White House press secretary Sarah Sanders said in a statement.
Bossert, 43, led the administration's efforts to protect the homeland from terrorist threats, strengthen cyber defenses, and respond to a series of natural disasters that have occurred across the country over the past year or so.
Sanders said that Trump "thanks him for his patriotic service and wishes him well," but she did not give a reason for Bossert's departure.
Bossert, who previously served in George W. Bush administration, was tapped to be assistant to the president for homeland security and counterterrorism in December, 2016.
He was a Trump favorite for his articulate TV style when defending the administration's agenda and made a name for himself as the public face of the administration's response to hurricanes that hit Texas, Florida and Puerto Rico last year.
Last weekend, Bossert also represented the administration on television to talks about the U.S. response to an alleged chemical weapons attack in Syria.
Bossert's resignation came a day after John Bolton took over as Trump's new national security adviser and two days after Michael Anton announced his departure as spokesman for the National Security Council.
Bolton, a former hawkish diplomat, is expected to add more momentum to the increasingly assertive U.S. foreign policy.
A source close to Bossert told NBC News that the adviser was unware of any intention at the White House to seek his resignation, and that he had no plans for the move.
"New team," the source said, without further explanation.
According to The New York Times, a White House official said that Rob Joyce, special assistant to the president and cybersecurity coordinator on the National Security Council, would take Bossert's place on an acting basis.
Bossert's resignation also marks the latest high-level departure from the White House.
Other Trump administration officials who have either resigned or been fired in the past few months include Secretary of State Rex Tillerson, National Security Adviser H.R. McMaster, National Economic Council Director Gary Cohn, Communications Director Hope Hicks, among others.