The top White House official responsible for Russia and Europe policy was placed on leave last week pending a security investigation, U.S. officials said Sunday.

Andrew Peek is a former State Department official who assumed the senior policy job at the White House National Security Council in the fall. He succeeded Fiona Hill, who testified in the House impeachment hearings that resulted in impeachment articles against President Trump.

News of Peek’s departure was first reported by Axios.

The White House and other officials would not comment on what led to Peek’s departure.

“We do not discuss personnel matters,” a spokesperson for the NSC said Sunday.

Peek had been expected to join national security adviser Robert C. O’Brien and Trump in Switzerland this week for the World Economic Forum meeting.