Donald Trump, who has already promised to use the Justice Department to “go after” his political adversaries, is expected to install Mr. Patel in a senior role if he returns to power.
The adviser, Kash Patel, had previously declined to answer questions from prosecutors in front of a federal grand jury, citing his Fifth Amendment rights.
The Justice Department is seeking to compel Kash Patel, an ally of the former president, to answer questions about assertions that material taken from the White House had been declassified.
The Justice Department’s effort to win the witnesses’ cooperation shows how the investigation stemming from the classified materials found at Mar-a-Lago is entering a new phase.