Former Trump White House official Ezra Cohen-Watnick is joining the Department of Justice as a national security adviser to Attorney General Jeff Sessions, a source confirmed on Wednesday.

His first day will be Monday, according to a source familiar with his appointment.

“Just like at the NSC, I expect Ezra to prioritize strengthening the United States’ posture against Russia, China and Iran as one of his top priorities,” the source said.

“With the Justice Department in charge of the domestic side of national security, he’ll find ways to make sure these countries do not threaten U.S. sovereignty.”

The appointment marks a return to the administration for the 31-year-old, who was first brought into the White House by former National Security Adviser Army Lt. Gen. (Ret.) Michael Flynn.

He served at the National Security Council as the senior director for intelligence programs, but resigned in August over policy differences with Flynn’s successor, Army Lt. Gen. H.R. McMaster. Since then, he has been working at tech company Oracle.

Cohen-Watnick’s appointment comes only days after the departure of McMaster, but had been discussed for months, according to the source.

Cohen-Watnick was blamed for giving House Intelligence Committee Chairman Devin Nunes (R-CA) classified information that proved the Obama administration had unmasked members of the Trump campaign, but it is not true, according to the source and the New York Times.

His return follows recent personnel changes that have shifted towards Trump loyalists.

Amb. John Bolton began as national security adviser on Monday, and has been mandated to stop leaks emanating from the NSC. He is also expected to purge those who are not on board on with Trump’s national security agenda.

This story has been updated.