A top adviser of Defense Secretary Pete Hegseth was escorted from the Pentagon on Tuesday as part of an investigation into an unauthorized disclosure of sensitive information, a U.S. defense official said.

Dan Caldwell, who was identified as part of an ongoing leak investigation, was placed on administrative leave, the official said, speaking on the condition of anonymity.

Since arriving at the Pentagon with Mr. Hegseth in January, Mr. Caldwell has accompanied the defense secretary to some of his meetings with key foreign leaders. In a Signal text chain first disclosed by The Atlantic last month, Mr. Caldwell was listed as Mr. Hegseth’s representative to the White House as it prepared to launch strikes in Yemen.

He also advised Mr. Hegseth on other key matters related to U.S. military operations around the world. The investigation into Mr. Caldwell and his removal from the Pentagon were reported earlier by Reuters.

Mr. Caldwell and Mr. Hegseth have a long relationship, dating back more than a decade to their tenure at Concerned Veterans for America, a conservative group that pressed for reforms to the Department of Veterans Affairs. Mr. Caldwell, a Marine Corps veteran, served in Iraq but eventually doubted the wisdom of the war.

“I think the Iraq war was a monstrous crime,” Mr. Caldwell told The Financial Times in December 2024. Mr. Hegseth has expressed similar reservations about the war.

Mr. Caldwell previously worked for Defense Priorities, a think tank that pushed for a more restrained U.S. foreign policy and was deeply skeptical of American engagement in the Middle East and support to Ukraine.

Mr. Caldwell’s positions have been a source of debate within his own Republican Party, which remains divided between those who maintain that the U.S. military is overextended in the world and traditionalist Republicans who see American power as central to global stability.

In recent weeks, the Trump administration and the Pentagon have aggressively pursued leak investigations. In March, Joe Kasper, Mr. Hegseth’s chief of staff, ordered an investigation into unauthorized disclosures of national security information and suggested that the Pentagon would begin administering polygraphs to senior officials.

Mr. Caldwell did not respond to a request for comment. Pentagon officials declined to comment on the details of the investigation.