Deputy chief of staff Darin Selnick is the second top Defense Department official -- along with Dan Caldwell, another senior advisor to Pentagon chief Pete Hegseth -- to be put on leave this week.

Hegseth's chief of staff Joe Kasper signed a March 21 memo requesting an investigation into "recent unauthorized disclosures of national security information involving sensitive communications."

"I expect to be informed immediately if this effort results in information identifying a party responsible for an unauthorized disclosure, and that such information will be referred to the appropriate criminal law enforcement entity for criminal prosecution," the memo said.

In a sign of his seniority, Caldwell was named by Hegseth as the Defense Department's point of contact in a group chat on commercial messaging app Signal in which top officials discussed military action against Yemen's Huthi rebels.

The Pentagon inspector general's office announced an investigation earlier this month into Hegseth's use of the app -- a probe unrelated to the one requested by Kasper.

The use of Signal by top officials resulted in a scandal when National Security Advisor Mike Waltz inadvertently added a journalist to the chat on the Yemen strikes, leading to the disclosure of its contents.

Neither Waltz nor Hegseth have been fired or otherwise disciplined over the incident so far.